LeetCode Java Solutions

Hi 👋, I'm Chee Hwa and welcome to LeetCode Solutions in Java.

As I'm new to LeetCode and programming in general, I'm practicing everyday and hope to share my solutions and learnings with you.

Exclusively on Java
Beginner problems
Daily Challenge if within my ability

Latest Solutions

Date	Problem	Difficulty	Solution	Topics
September 14th	1775. Equal Sum Arrays With Minimum Number of Operations	$\text{\color{Dandelion}Medium}$	Counting	Array, Counting, Greedy
September 13th	1814. Count Nice Pairs in an Array	$\text{\color{Dandelion}Medium}$	Counting & Hash Table	Array, Counting, Hash Table, Math
September 12th	2006. Count Number of Pairs With Absolute Difference K	$\text{\color{TealBlue}Easy}$	Hash Table	Array, Hash Table
September 11th	2006. Count Number of Pairs With Absolute Difference K	$\text{\color{TealBlue}Easy}$	Counting	Array, Counting
September 10th	2006. Count Number of Pairs With Absolute Difference K	$\text{\color{TealBlue}Easy}$	Brute Force	Array

Topics

The problem sets and solutions are grouped into Topics as stated in LeetCode.

Array

#	Problem	Solution	Difficulty	Topics	Note
1	Two Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
15	3Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Sorting, Two Pointers
16	3Sum Closest	Java	$\text{\color{Dandelion}Medium}$	Array, Sorting, Two Pointers
26	Remove Duplicates from Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
27	Remove Element	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
36	Valid Sudoku	Java without or with Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
37	Sudoku Solver	Java	$\text{\color{Red}Hard}$	Array, Backtracking, Matrix
45	Jump Game II	Dynamic Programming (Memoization), Dynamic Programming (Tabulation) or Greedy	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy
46	Permutations	Java with Backtracking, Iteration or Recursion	$\text{\color{Dandelion}Medium}$	Array, Backtracking
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
55	Jump Game	Java with Dynamic Programming or Greedy	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy
57	Insert Interval	Java	$\text{\color{Dandelion}Medium}$	Array
73	Set Matrix Zeroes	Java with Array, Hash Table or In-Place	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
79	Word Search	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Matrix
80	Remove Duplicates from Sorted Array II	Java	$\text{\color{Dandelion}Medium}$	Array, Two Pointers
88	Merge Sorted Array	Java with Sorting or Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
121	Best Time to Buy and Sell Stock	Java with or without Dynamic Programming	$\text{\color{TealBlue}Easy}$	Array, Dynamic Programming
130	Surrounded Regions	Java with Breadth-First Search, Depth-First Search or Union Find	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find
134	Gas Station	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy
136	Single Number	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation	Stream Reduce
152	Maximum Product Subarray	Java with Array or Dynamic Programming	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming
167	Two Sum II - Input Array is Sorted	Java with Binary Search or Two Pointers	$\text{\color{Dandelion}Medium}$	Array, Binary Search, Two Pointers
169	Majority Element	Java with Boyer-Moore Majority Voting Algorithm, Counting & Divide and Conquer, Counting & Hash Table or Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting, Divide and Conquer, Hash Table, Sorting
200	Number of Islands	Java with Breadth-First Search, Depth-First Search or Union Find	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find
217	Contains Duplicate	Java with Hash Map, Hash Set or Sorting	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sorting
219	Contains Duplicate II	Java with Hash Map or Sliding Window	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sliding Window
220	Contains Duplicate III	Java with Sliding Window & Bucket Sort, Sliding Window & Ordered Set or Sorting	$\text{\color{Red}Hard}$	Array, Bucket Sort, Ordered Set, Sliding Window, Sorting
228	Summary Ranges	Java	$\text{\color{TealBlue}Easy}$	Array
229	Majority Element II	Java with Boyer-Moore Majority Voting Algorithm, Counting & Hash Table or Sorting	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting
238	Product of Array Except Self	Java	$\text{\color{Dandelion}Medium}$	Array, Prefix Sum
274	H-Index	Java with Counting Sort and Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Sorting
275	H-Index II	Java	$\text{\color{Dandelion}Medium}$	Array, Binary Search
283	Move Zeroes	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
334	Increasing Triplet Subsequence	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
393	UTF-8 Validation	Java without or with Bit Manipulation	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation
414	Third Maximum Number	Java with Sorting, without Sorting or with Hash Table & Priority Queue	$\text{\color{TealBlue}Easy}$	Array, Sorting
448	Find All Numbers Disappeared in an Array	Java with or without Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table
485	Maximum Consecutive Ones	Java	$\text{\color{TealBlue}Easy}$	Array
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
503	Next Greater Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Monotonic Stack, Stack
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
560	Subarray Sum Equals K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
561	Array Partition	Java with Greedy & Counting Sort or Greedy & Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Greedy, Sorting
609	Find Duplicate File in System	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, String
645	Set Mismatch	Java with Bit Manipulation, Hash Table or Sorting	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting
713	Subarray Product Less Than K	Java	$\text{\color{Dandelion}Medium}$	Array, Sliding Window
744	Find Smallest Letter Greater Than Target	Java with Linear Search or Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search
746	Min Cost Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Array, Dynamic Programming
766	Toeplitz Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix	Follow up: One Row at a Time and One Column at a Time
812	Largest Triangle Area	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
835	Image Overlap	Java with Bit Manipulation or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Matrix
904	Fruit Into Baskets	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sliding Window
905	Sort Array By Parity	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
922	Sort Array By Parity II	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
923	3Sum With Multiplicity	Java with Counting Hash Table or Sorting & Two Pointers	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting, Two Pointers
929	Unique Email Addresses	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
941	Valid Mountain Array	Java	$\text{\color{TealBlue}Easy}$	Array
944	Delete Columns to Make Sorted	Java	$\text{\color{TealBlue}Easy}$	Array, String
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
953	Verifying an Alien Dictionary	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
974	Subarray Sums Divisible by K	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
977	Squares of a Sorted Array	Java with Sorting or Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
985	Sum of Even Numbers After Queries	Java	$\text{\color{Dandelion}Medium}$	Array, Simulation
989	Add to Array-Form of Integer	Java	$\text{\color{TealBlue}Easy}$	Array, Math
1019	Next Greater Node In Linked List	Java	$\text{\color{Dandelion}Medium}$	Array, Linked List, Monotonic Stack, Stack
1037	Valid Boomerang	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1051	Height Checker	Java with Counting Sort or Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Sorting
1089	Duplicate Zeros	Java without or with Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
1122	Relative Sort Array	Java with Counting Sort or Hash Table	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Hash Table, Sorting
1207	Unique Number of Occurrences	Java with Counting & Sorting or Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table
1232	Check If It Is a Straight Line	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1239	Maximum Length of a Concatenated String with Unique Characters	Java with Backtracking or Bit Manipulation	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String
1295	Find Numbers with Even Number of Digits	Java	$\text{\color{TealBlue}Easy}$	Array
1299	Replace Elements with Greatest Element on Right Side	Java	$\text{\color{TealBlue}Easy}$	Array
1337	The K Weakest Rows in a Matrix	Java with Linear Search, Binary Search, Heap (Priority Queue) & Binary Search or Sorting & Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting
1346	Check If N and Its Double Exist	Java with Binary Search & Sorting, Hash Table or Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Hash Table, Sorting, Two Pointers
1351	Count Negative Numbers in a Sorted Matrix	Java with Linear Search or Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Matrix
1470	Shuffle the Array	Java	$\text{\color{TealBlue}Easy}$	Array
1480	Running Sum of 1D Array	Java	$\text{\color{TealBlue}Easy}$	Array, Prefix Sum
1497	Check If Array Pairs Are Divisible by k	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table
1534	Count Good Triplets	Java	$\text{\color{TealBlue}Easy}$	Array, Enumeration
1578	Minimum Time to Make Rope Colorful	Java with Dynamic Programming or Greedy	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String
1636	Sort Array by Increasing Frequency	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sorting
1662	Check If Two String Arrays are Equivalent	Java with or without Pointers	$\text{\color{TealBlue}Easy}$	Array, String
1672	Richest Customer Wealth	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix
1679	Max Number of K-Sum Pairs	Java with Hash Table or Sorting & Two Pointers	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers
1711	Count Good Meals	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table
1732	Find the Highest Altitude	Java	$\text{\color{TealBlue}Easy}$	Array, Prefix Sum
1775	Equal Sum Arrays With Minimum Number of Operations	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy
1814	Count Nice Pairs in an Array	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Math
1833	Maximum Ice Cream Bars	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting
1834	Single-Threaded CPU	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue), Sorting
1865	Finding Pairs With a Certain Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Design, Hash Table
1920	Build Array from Permutation	Java	$\text{\color{TealBlue}Easy}$	Array, Simulation
1926	Nearest Exit from Entrance in Maze	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Matrix
1962	Remove Stones to Minimize the Total	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue)
1979	Find Greatest Common Divisor of Array	Java	$\text{\color{TealBlue}Easy}$	Array, Math, Number Theory
1995	Count Special Quadruplets	Java	$\text{\color{TealBlue}Easy}$	Array, Enumeration	Solution Using Hash Table
2006	Count Number of Pairs With Absolute Difference K	Java with Brute Force, Counting or Hash Table	$\text{\color{TealBlue}Easy}$	Array, Counting, Hash Table
2007	Find Original Array From Doubled Array	Java with Hash Table, Queue or Counting Array	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting
2023	Number of Pairs of Strings With Concatenation Equal to Target	Java	$\text{\color{Dandelion}Medium}$	Array, String
2028	Find Missing Observations	Java	$\text{\color{Dandelion}Medium}$	Array, Math, Simulation
2131	Longest Palindrome by Concatenating Two Letter Words	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String
2133	Check if Every Row and Column Contains All Numbers	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Matrix
2136	Earliest Possible Day of Full Bloom	Java	$\text{\color{Red}Hard}$	Array, Greedy, Sorting
2176	Count Equal and Divisible Pairs in an Array	Java	$\text{\color{TealBlue}Easy}$	Array
2244	Minimum Rounds to Complete All Tasks	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table
2279	Maximum Bags With Full Capacity of Rocks	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting
2352	Equal Row and Column Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix, Simulation
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting
2395	Find Subarrays With Equal Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
2399	Check Distances Between Same Letters	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
2438	Range Product Queries of Powers	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation, Prefix Sum
2452	Words Within Two Edits of Dictionary	Java	$\text{\color{Dandelion}Medium}$	Array, String
2453	Destroy Sequential Targets	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table
2454	Next Greater Element IV	Java	$\text{\color{Red}Hard}$	Array, Monotonic Stack, Stack

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Backtracking

#	Problem	Solution	Difficulty	Topics	Note
37	Sudoku Solver	Java	$\text{\color{Red}Hard}$	Array, Backtracking, Matrix
46	Permutations	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking	Solution Using Iteration or Recursion
79	Word Search	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Matrix
113	Path Sum II	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Binary Tree, Depth-First Search, Tree
257	Binary Tree Paths	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Backtracking, Binary Tree, Depth-First Search, String, Tree
797	All Paths From Source to Target	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph	Solution Using Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)
967	Numbers With Same Consecutive Differences	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search	Solution Using Breadth-First Search
1239	Maximum Length of a Concatenated String with Unique Characters	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String	Solution Using Bit Manipulation

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Binary Search

#	Problem	Solution	Difficulty	Topics	Note
102	Binary Tree Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
103	Binary Tree Zigzag Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
107	Binary Tree Level Order Traversal II	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
144	Binary Tree Preorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree
145	Binary Tree Postorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree
167	Two Sum II - Input Array is Sorted	Java	$\text{\color{Dandelion}Medium}$	Array, Binary Search, Two Pointers	Solution Using Two Pointers
222	Count Complete Tree Nodes	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Binary Tree, Depth-First Search, Tree
275	H-Index II	Java	$\text{\color{Dandelion}Medium}$	Array, Binary Search
374	Guess Number Higher or Lower	Java	$\text{\color{TealBlue}Easy}$	Binary Search, Interactive
744	Find Smallest Letter Greater Than Target	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search	Solution Using Linear Search
1337	The K Weakest Rows in a Matrix	Java with Binary Search, Heap (Priority Queue) & Binary Search or Sorting & Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting	Solution Using Linear Search
1346	Check If N and Its Double Exist	Java with Binary Search & Sorting	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Hash Table, Sorting, Two Pointers	Solution Using Hash Table or Two Pointers
1351	Count Negative Numbers in a Sorted Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Matrix	Solution Using Linear Search
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting

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Binary Search Tree

#	Problem	Solution	Difficulty	Topics
98	Validate Binary Search Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Search Tree, Binary Tree, Depth-First Search, Tree
653	Two Sum IV - Input is a BST	Java with Breadth-First Search (with Hash Table), Breadth-First Search (without Hash Table), Depth-First Search (Iterative), Depth-First Search (Recursive), Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers
783	Minimum Distance Between BST Nodes	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Tree

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Binary Tree

#	Problem	Solution	Difficulty	Topics
94	Binary Tree Inorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Stack, Tree
98	Validate Binary Search Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Search Tree, Binary Tree, Depth-First Search, Tree
100	Same Tree	Java with Breadth-First Search or Depth-First Search	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
104	Maximum Depth of Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
110	Balanced Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Tree
111	Minimum Depth of Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
112	Path Sum	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
113	Path Sum II	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Binary Tree, Depth-First Search, Tree
114	Flatten Binary Tree to Linked List	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Linked List, Stack, Tree
129	Sum Root to Leaf Numbers	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
222	Count Complete Tree Nodes	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Binary Tree, Depth-First Search, Tree
226	Invert Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
257	Binary Tree Paths	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Backtracking, Binary Tree, Depth-First Search, String, Tree
404	Sum of Left Leaves	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
437	Path Sum III	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
653	Two Sum IV - Input is a BST	Java with Breadth-First Search (with Hash Table), Breadth-First Search (without Hash Table), Depth-First Search (Iterative), Depth-First Search (Recursive), Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers
783	Minimum Distance Between BST Nodes	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Tree
988	Smallest String Starting From Leaf	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, String, Tree
1026	Maximum Difference Between Node and Ancestor	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
1457	Pseudo-Palindromic Paths in a Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Bit Manipulation, Breadth-First Search, Depth-First Search, Tree

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Bit Manipulation

#	Problem	Solution	Difficulty	Topics	Note
29	Divide Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
136	Single Number	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation	Bonus Solution Using Stream Reduce
371	Sum of Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
393	UTF-8 Validation	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation	Solution without using Bit Manipulation
645	Set Mismatch	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting	Solution Using Hash Table or Sorting
1239	Maximum Length of a Concatenated String with Unique Characters	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String	Solution Using Backtracking
1318	Minimum Flips to Make a OR b Equal to c	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation
1342	Number of Steps to Reduce a Number to Zero	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math	Solution Using Math
1457	Pseudo-Palindromic Paths in a Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Bit Manipulation, Breadth-First Search, Depth-First Search, Tree
1486	XOR Operation in an Array	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math
1680	Concatenation of Consecutive Binary Numbers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math, Simulation	Solution Using Math
2220	Minimum Bit Flips to Convert Number	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation
2438	Range Product Queries of Powers	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation, Prefix Sum

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Brainteaser

#	Problem	Solution	Difficulty	Topics	Note
1033	Moving Stones Until Consecutive	Java	$\text{\color{Dandelion}Medium}$	Brainteaser, Math

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Breadth-First Search

#	Problem	Solution	Difficulty	Topics	Note
100	Same Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search
102	Binary Tree Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
103	Binary Tree Zigzag Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
104	Maximum Depth of Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
107	Binary Tree Level Order Traversal II	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
111	Minimum Depth of Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
112	Path Sum	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
130	Surrounded Regions	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Depth-First Search or Union Find
200	Number of Islands	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Depth-First Search or Union Find
226	Invert Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
404	Sum of Left Leaves	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
429	N-ary Tree Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Breadth-First Search, Tree
559	Maximum Depth of Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
653	Two Sum IV - Input is a BST	Java with or without Hash Table	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers	Solution Using Depth-First Search (Iterative), Depth-First Search (Recursive), Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)
783	Minimum Distance Between BST Nodes	Java	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
797	All Paths From Source to Target	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph	Solution Using Depth-First Search (Backtracking), Depth-First Search (Iterative) or Depth-First Search (Recursive)
967	Numbers With Same Consecutive Differences	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search	Solution Using Backtracking
1457	Pseudo-Palindromic Paths in a Binary Tree	Java	$\text{\color{Dandelion}Medium}$	Binary Tree, Bit Manipulation, Breadth-First Search, Depth-First Search, Tree	Solution Using Depth-First Search (Iterative) or Depth-First Search (Recursive)
1926	Nearest Exit from Entrance in Maze	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Matrix
1971	Find if Path Exists in Graph	Java	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find	Solution Using Depth-First Search (Iterative), Depth-First Search (Recursive) or Union Find

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Bucket Sort

#	Problem	Solution	Difficulty	Topics	Note
220	Contains Duplicate III	Java with Sliding Window & Bucket Sort	$\text{\color{Red}Hard}$	Array, Bucket Sort, Ordered Set, Sliding Window, Sorting	Solution Using Sliding Window & Ordered Set or Sorting

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Counting

#	Problem	Solution	Difficulty	Topics	Note
169	Majority Element	Java with Counting & Divide and Conquer or Counting & Hash Table	$\text{\color{TealBlue}Easy}$	Array, Counting, Divide and Conquer, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm or Sorting
229	Majority Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm or Sorting
383	Ransom Note	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Hash Table
923	3Sum With Multiplicity	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting, Two Pointers	Solution Using Hash Table or Sorting & Two Pointers
1497	Check If Array Pairs Are Divisible by k	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table	Solution Using Hash Table
1603	Design Parking System	Java	$\text{\color{TealBlue}Easy}$	Counting, Design, Simulation
1704	Determine if String Halves Are Alike	Java	$\text{\color{TealBlue}Easy}$	Counting, String
1775	Equal Sum Arrays With Minimum Number of Operations	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy
1814	Count Nice Pairs in an Array	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Math
2006	Count Number of Pairs With Absolute Difference K	Java	$\text{\color{TealBlue}Easy}$	Array, Counting, Hash Table	Solution Using Brute Force Approach or Hash Table
2131	Longest Palindrome by Concatenating Two Letter Words	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String	Solution Using Hash Table
2244	Minimum Rounds to Complete All Tasks	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table
2351	First Letter to Appear Twice	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Hash Table
2453	Destroy Sequential Targets	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table

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Counting Sort

#	Problem	Solution	Difficulty	Topics	Note
274	H-Index	Java	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Sorting	Solution Using Sorting
561	Array Partition	Java	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Greedy, Sorting	Solution Using Greedy & Sorting
1051	Height Checker	Java	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Sorting	Solution Using Sorting
1122	Relative Sort Array	Java	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Hash Table, Sorting	Solution Using Hash Table

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Data Stream

#	Problem	Solution	Difficulty	Topics	Note
295	Find Median from Data Stream	Java	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers

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Depth-First Search

#	Problem	Solution	Difficulty	Topics	Note
94	Binary Tree Inorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Stack, Tree
98	Validate Binary Search Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Search Tree, Binary Tree, Depth-First Search, Tree
100	Same Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
104	Maximum Depth of Binary Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
110	Balanced Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Tree
111	Minimum Depth of Binary Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
112	Path Sum	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
113	Path Sum II	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Binary Tree, Depth-First Search, Tree
114	Flatten Binary Tree to Linked List	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Linked List, Stack, Tree
129	Sum Root to Leaf Numbers	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
130	Surrounded Regions	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Breadth-First Search or Union Find
144	Binary Tree Preorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree
145	Binary Tree Postorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree
200	Number of Islands	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Breadth-First Search or Union Find
222	Count Complete Tree Nodes	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Binary Tree, Depth-First Search, Tree
226	Invert Binary Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
257	Binary Tree Paths	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Backtracking, Binary Tree, Depth-First Search, String, Tree
404	Sum of Left Leaves	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
437	Path Sum III	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
559	Maximum Depth of Binary Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
653	Two Sum IV - Input is a BST	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers	Solution Using Breadth-First Search (with Hash Table), Breadth-First Search (without Hash Table), Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)
783	Minimum Distance Between BST Nodes	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
797	All Paths From Source to Target	Java with Depth-First Search (Backtracking), Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph	Solution Using Breadth-First Search
988	Smallest String Starting From Leaf	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, String, Tree
1026	Maximum Difference Between Node and Ancestor	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
1457	Pseudo-Palindromic Paths in a Binary Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Bit Manipulation, Breadth-First Search, Depth-First Search, Tree	Solution Using Breadth-First Search
1971	Find if Path Exists in Graph	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find	Solution Using Breadth-First Search or Union Find
2359	Find Closest Node to Given Two Nodes	Java	$\text{\color{Dandelion}Medium}$	Depth-First Search, Graph

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Design

#	Problem	Solution	Difficulty	Topics
225	Implement Stack using Queues	Java	$\text{\color{TealBlue}Easy}$	Design, Queue, Stack
232	Implement Queue using Stacks	Java	$\text{\color{TealBlue}Easy}$	Design, Queue, Stack
295	Find Median from Data Stream	Java	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers
707	Design Linked List	Java with Singly Linked List or Doubly Linked List	$\text{\color{Dandelion}Medium}$	Design, Linked List
1603	Design Parking System	Java	$\text{\color{TealBlue}Easy}$	Counting, Design, Simulation
1865	Finding Pairs With a Certain Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Design, Hash Table

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Divide and Conquer

#	Problem	Solution	Difficulty	Topics	Note
169	Majority Element	Java	$\text{\color{TealBlue}Easy}$	Array, Counting, Divide and Conquer, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm, Counting & Hash Table or Sorting

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Dynamic Programming

#	Problem	Solution	Difficulty	Topics	Note
5	Longest Palindromic Substring	Java	$\text{\color{Dandelion}Medium}$	Dynamic Programming, String	Solution Using Two Pointers with Approach 1 or Approach 2
45	Jump Game II	Java with Dynamic Programming - Memoization or Tabulation	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy	Solution Using Greedy
55	Jump Game	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy	Solution Using Greedy
70	Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
121	Best Time to Buy and Sell Stock	Java	$\text{\color{TealBlue}Easy}$	Array, Dynamic Programming	Solution without Using Dynamic Programming
152	Maximum Product Subarray	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming	Solution Using Array
264	Ugly Number II	Java	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math	Solution Using Heap (Priority Queue)
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion
746	Min Cost Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Array, Dynamic Programming
1137	N-th Tribonacci Number	Java with Dynamic Programming Iteratively or Recursively (Memoization)	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
1155	Number of Dice Rolls With Target Sum	Java with Dynamic Programming using Memoization or Tabulation	$\text{\color{Dandelion}Medium}$	Dynamic Programming
1578	Minimum Time to Make Rope Colorful	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String	Solution Using Greedy

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Enumeration

#	Problem	Solution	Difficulty	Topics	Note
1534	Count Good Triplets	Java	$\text{\color{TealBlue}Easy}$	Array, Enumeration
1995	Count Special Quadruplets	Java	$\text{\color{TealBlue}Easy}$	Array, Enumeration	Solution Using Hash Table
2240	Number of Ways to Buy Pens and Pencils	Java	$\text{\color{Dandelion}Medium}$	Enumeration, Math
2437	Number of Valid Clock Times	Java	$\text{\color{TealBlue}Easy}$	Enumeration, String

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Geometry

#	Problem	Solution	Difficulty	Topics
223	Rectangle Area	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
593	Valid Square	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
812	Largest Triangle Area	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
836	Rectangle Overlap	Java	$\text{\color{TealBlue}Easy}$	Geometry, Math
1037	Valid Boomerang	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1232	Check If It Is a Straight Line	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math

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Graph

#	Problem	Solution	Difficulty	Topics
797	All Paths From Source to Target	Java with Breadth-First Search, Depth-First Search (Backtracking), Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph
1971	Find if Path Exists in Graph	Java with Breadth-First Search, Depth-First Search (Iterative), Depth-First Search (Recursive) or Union Find	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find
2359	Find Closest Node to Given Two Nodes	Java	$\text{\color{Dandelion}Medium}$	Depth-First Search, Graph

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Greedy

#	Problem	Solution	Difficulty	Topics	Note
45	Jump Game II	Java with Greedy	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy	Solution Using Dynamic Programming (Memoization) or Dynamic Programming (Tabulation)
55	Jump Game	Java with Greedy	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy	Solution Using Dynamic Programming
134	Gas Station	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy
334	Increasing Triplet Subsequence	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy
561	Array Partition	Java with Greedy & Counting Sort or Greedy & Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Greedy, Sorting
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
1323	Maximum 69 Number	Java with Math or String	$\text{\color{TealBlue}Easy}$	Greedy, Math
1328	Break a Palindrome	Java	$\text{\color{Dandelion}Medium}$	Greedy, String
1578	Minimum Time to Make Rope Colorful	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String	Solution Using Dynamic Programming
1775	Equal Sum Arrays With Minimum Number of Operations	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy
1833	Maximum Ice Cream Bars	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting
2007	Find Original Array From Doubled Array	Java with Hash Table, Queue or Counting Array	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting
2131	Longest Palindrome by Concatenating Two Letter Words	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String
2136	Earliest Possible Day of Full Bloom	Java	$\text{\color{Red}Hard}$	Array, Greedy, Sorting
2244	Minimum Rounds to Complete All Tasks	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table
2279	Maximum Bags With Full Capacity of Rocks	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting

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Hash Table

#	Problem	Solution	Difficulty	Topics	Note
1	Two Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
3	Longest Substring Without Repeating Characters	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String	Solution without Using Hash Table
12	Integer to Roman	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Math, String	Solution without Hash Table
13	Roman to Integer	Java	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String	Solution without Hash Table
36	Valid Sudoku	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix	Solution without Hash Table
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
73	Set Matrix Zeroes	Java with Array, Hash Table or In-Place	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
169	Majority Element	Java	$\text{\color{TealBlue}Easy}$	Array, Counting, Divide and Conquer, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm, Counting & Divide and Conquer or Sorting
205	Isomorphic Strings	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
217	Contains Duplicate	Java with Hash Map or Hash Set	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sorting	Solution Using Sorting
219	Contains Duplicate II	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sliding Window	Solution Using Sliding Window
229	Majority Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm or Sorting
264	Ugly Number II	Java with Dynamic Programming or Heap (Priority Queue)	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math
290	Word Pattern	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
383	Ransom Note	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Counting Array
438	Find All Anagrams in a String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String
448	Find All Numbers Disappeared in an Array	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table	Solution without Hash Table
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
560	Subarray Sum Equals K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
567	Permutation in String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String, Two Pointers
609	Find Duplicate File in System	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, String
645	Set Mismatch	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting	Solution Using Bit Manipulation or Sorting
653	Two Sum IV - Input is a BST	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers	Solution Using Breadth-First Search (without Hash Table), Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)
904	Fruit Into Baskets	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sliding Window
923	3Sum With Multiplicity	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting, Two Pointers	Solution Using Counting or Sorting & Two Pointers
929	Unique Email Addresses	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
953	Verifying an Alien Dictionary	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
974	Subarray Sums Divisible by K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum	Solution Using Counting
1122	Relative Sort Array	Java	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Hash Table, Sorting	Solution Using Counting Sort
1207	Unique Number of Occurrences	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table	Solution Using Counting & Sorting
1346	Check If N and Its Double Exist	Java with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Hash Table, Sorting, Two Pointers	Solution Using Binary Search & Sorting or Two Pointers
1497	Check If Array Pairs Are Divisible by k	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table	Solution Using Counting
1636	Sort Array by Increasing Frequency	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sorting
1657	Determine if Two Strings Are Close	Java with Hash Table & Sorting	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting	Solution Using only Sorting
1679	Max Number of K-Sum Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers	Solution Using Sorting & Two Pointers
1711	Count Good Meals	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table
1814	Count Nice Pairs in an Array	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Math
1832	Check if the Sentence is Pangram	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String	Solution Using Bit Manipulation , Counting and Without Hash Table
1865	Finding Pairs With a Certain Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Design, Hash Table
1935	Maximum Number of Words You Can Type	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
2006	Count Number of Pairs With Absolute Difference K	Java	$\text{\color{TealBlue}Easy}$	Array, Counting, Hash Table	Solution Using Brute Force, Counting
2007	Find Original Array From Doubled Array	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting	Solution Using Queue or Counting Array
2131	Longest Palindrome by Concatenating Two Letter Words	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String	Solution Using Counting
2133	Check if Every Row and Column Contains All Numbers	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Matrix	Solution Without Hash Table
2244	Minimum Rounds to Complete All Tasks	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table
2351	First Letter to Appear Twice	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Counting
2352	Equal Row and Column Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix, Simulation
2395	Find Subarrays With Equal Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
2399	Check Distances Between Same Letters	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String	Solution Without Hash Table
2451	Odd String Difference	Java	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String	Solution Without Hash Table
2453	Destroy Sequential Targets	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table

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Heap (Priority Queue)

#	Problem	Solution	Difficulty	Topics	Note
264	Ugly Number II	Java	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math	Solution Using Dynamic Programming
295	Find Median from Data Stream	Java	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers
1337	The K Weakest Rows in a Matrix	Java with Heap (Priority Queue) & Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting	Solution Using Linear Search, Binary Search, Sorting & Binary Search
1834	Single-Threaded CPU	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue), Sorting
1962	Remove Stones to Minimize the Total	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue)

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Interactive

#	Problem	Solution	Difficulty	Topics	Note
374	Guess Number Higher or Lower	Java	$\text{\color{TealBlue}Easy}$	Binary Search, Interactive

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Linked List

#	Problem	Solution	Difficulty	Topics
19	Remove Nth Node From End of List	Java with One Pass or Two Passes	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers
24	Swap Nodes in Pairs	Java with Iteration or Recursion	$\text{\color{Dandelion}Medium}$	Linked List, Recursion
92	Reverse Linked List II	Java	$\text{\color{Dandelion}Medium}$	Linked List
114	Flatten Binary Tree to Linked List	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Linked List, Stack, Tree
206	Reverse Linked List	Java with Array List, Reverse Nodes Iteratively or Recursively	$\text{\color{TealBlue}Easy}$	Linked List, Recursion
707	Design Linked List	Java with Singly Linked List or Doubly Linked List	$\text{\color{Dandelion}Medium}$	Design, Linked List
876	Middle of the Linked List	Java	$\text{\color{TealBlue}Easy}$	Linked List, Two Pointers
1019	Next Greater Node In Linked List	Java	$\text{\color{Dandelion}Medium}$	Array, Linked List, Monotonic Stack, Stack
2095	Delete the Middle Node of a Linked List	Java	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers
2130	Maximum Twin Sum of a Linked List	Java with Two Pointers or Two Pointers & Stack	$\text{\color{Dandelion}Medium}$	Linked List, Stack, Two Pointers

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Math

#	Problem	Solution	Difficulty	Topics	Note
12	Integer to Roman	Java with or without Hash Table	$\text{\color{Dandelion}Medium}$	Hash Table, Math, String
13	Roman to Integer	Java with or without Hash Table	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String
29	Divide Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
70	Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
223	Rectangle Area	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
263	Ugly Number	Java	$\text{\color{TealBlue}Easy}$	Math
264	Ugly Number II	Java with Dynamic Programming or Heap (Priority Queue)	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
371	Sum of Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
412	Fizz Buzz	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation, String
492	Construct the Rectangle	Java	$\text{\color{TealBlue}Easy}$	Math
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
556	Next Greater Element III	Java	$\text{\color{Dandelion}Medium}$	Math, String, Two Pointers
593	Valid Square	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
812	Largest Triangle Area	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
836	Rectangle Overlap	Java	$\text{\color{TealBlue}Easy}$	Geometry, Math
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
989	Add to Array-Form of Integer	Java	$\text{\color{TealBlue}Easy}$	Array, Math
1033	Moving Stones Until Consecutive	Java	$\text{\color{Dandelion}Medium}$	Brainteaser, Math
1037	Valid Boomerang	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1071	Greatest Common Divisor of Strings	Java	$\text{\color{TealBlue}Easy}$	Math, String
1103	Distribute Candies to People	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
1137	N-th Tribonacci Number	Java with Dynamic Programming Iteratively or Recursively (Memoization)	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
1154	Day of the Year	Java	$\text{\color{TealBlue}Easy}$	Math, String
1232	Check If It Is a Straight Line	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1323	Maximum 69 Number	Java	$\text{\color{TealBlue}Easy}$	Greedy, Math	Solution Using String
1342	Number of Steps to Reduce a Number to Zero	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math	Solution Using Bit Manipulation
1486	XOR Operation in an Array	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math
1523	Count Odd Numbers in an Interval Range	Java	$\text{\color{TealBlue}Easy}$	Math
1680	Concatenation of Consecutive Binary Numbers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math, Simulation	Solution Using Bit Manipulation
1814	Count Nice Pairs in an Array	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Math
1979	Find Greatest Common Divisor of Array	Java	$\text{\color{TealBlue}Easy}$	Array, Math, Number Theory
2028	Find Missing Observations	Java	$\text{\color{Dandelion}Medium}$	Array, Math, Simulation
2169	Count Operations to Obtain Zero	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
2240	Number of Ways to Buy Pens and Pencils	Java	$\text{\color{Dandelion}Medium}$	Enumeration, Math
2413	Smallest Even Multiple	Java	$\text{\color{TealBlue}Easy}$	Math, Number Theory
2451	Odd String Difference	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String

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Matrix

#	Problem	Solution	Difficulty	Topics	Note
36	Valid Sudoku	Java without or with Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
37	Sudoku Solver	Java	$\text{\color{Red}Hard}$	Array, Backtracking, Matrix
73	Set Matrix Zeroes	Java with Array, Hash Table or In-Place	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
79	Word Search	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Matrix
130	Surrounded Regions	Java with Breadth-First Search, Depth-First Search or Union Find	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find
200	Number of Islands	Java with Breadth-First Search, Depth-First Search or Union Find	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find
766	Toeplitz Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix	Follow up: One Row at a Time and One Column at a Time
835	Image Overlap	Java with Bit Manipulation or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Matrix
1337	The K Weakest Rows in a Matrix	Java with Linear Search, Binary Search, Heap (Priority Queue) or Sorting	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting
1351	Count Negative Numbers in a Sorted Matrix	Java with Linear Search or Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Matrix
1672	Richest Customer Wealth	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix
1926	Nearest Exit from Entrance in Maze	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Matrix
2133	Check if Every Row and Column Contains All Numbers	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Matrix
2352	Equal Row and Column Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix, Simulation

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Memoization

#	Problem	Solution	Difficulty	Topics	Note
70	Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion
1137	N-th Tribonacci Number	Java	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization	Solution Using Dynamic Programming (Iterative)

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Monotonic Stack

#	Problem	Solution	Difficulty	Topics
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
503	Next Greater Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Monotonic Stack, Stack
1019	Next Greater Node In Linked List	Java	$\text{\color{Dandelion}Medium}$	Array, Linked List, Monotonic Stack, Stack
2454	Next Greater Element IV	Java	$\text{\color{Red}Hard}$	Array, Monotonic Stack, Stack

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Number Theory

#	Problem	Solution	Difficulty	Topics	Note
1979	Find Greatest Common Divisor of Array	Java	$\text{\color{TealBlue}Easy}$	Array, Math, Number Theory
2413	Smallest Even Multiple	Java	$\text{\color{TealBlue}Easy}$	Math, Number Theory

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Ordered Set

#	Problem	Solution	Difficulty	Topics	Note
220	Contains Duplicate III	Java with Sliding Window & Ordered Set	$\text{\color{Red}Hard}$	Array, Bucket Sort, Ordered Set, Sliding Window, Sorting	Solution Using Sliding Window & Bucket Sort or Sorting

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Prefix Sum

#	Problem	Solution	Difficulty	Topics
238	Product of Array Except Self	Java	$\text{\color{Dandelion}Medium}$	Array, Prefix Sum
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
560	Subarray Sum Equals K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
974	Subarray Sums Divisible by K	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
1480	Running Sum of 1D Array	Java	$\text{\color{TealBlue}Easy}$	Array, Prefix Sum
1732	Find the Highest Altitude	Java	$\text{\color{TealBlue}Easy}$	Array, Prefix Sum
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting
2438	Range Product Queries of Powers	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation, Prefix Sum

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Queue

#	Problem	Solution	Difficulty	Topics	Note
225	Implement Stack using Queues	Java	$\text{\color{TealBlue}Easy}$	Design, Queue, Stack
232	Implement Queue using Stacks	Java	$\text{\color{TealBlue}Easy}$	Design, Queue, Stack

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Recursion

#	Problem	Solution	Difficulty	Topics	Note
24	Swap Nodes in Pairs	Java	$\text{\color{Dandelion}Medium}$	Linked List, Recursion	Solution Using Iteration
206	Reverse Linked List	Java	$\text{\color{TealBlue}Easy}$	Linked List, Recursion	Solution Using Array List or Reverse Nodes Iteratively
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion

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Simulation

#	Problem	Solution	Difficulty	Topics
412	Fizz Buzz	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation, String
985	Sum of Even Numbers After Queries	Java	$\text{\color{Dandelion}Medium}$	Array, Simulation
1103	Distribute Candies to People	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
1603	Design Parking System	Java	$\text{\color{TealBlue}Easy}$	Counting, Design, Simulation
1680	Concatenation of Consecutive Binary Numbers	Java with Bit Manipulation or Math	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math, Simulation
1920	Build Array from Permutation	Java	$\text{\color{TealBlue}Easy}$	Array, Simulation
2028	Find Missing Observations	Java	$\text{\color{Dandelion}Medium}$	Array, Math, Simulation
2169	Count Operations to Obtain Zero	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
2352	Equal Row and Column Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix, Simulation

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Sliding Window

#	Problem	Solution	Difficulty	Topics	Note
3	Longest Substring Without Repeating Characters	Java with or without Hash Table	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String
219	Contains Duplicate II	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sliding Window	Solution Using Hash Table
220	Contains Duplicate III	Java with Sliding Window & Bucket Sort or Sliding Window & Ordered Set	$\text{\color{Red}Hard}$	Array, Bucket Sort, Ordered Set, Sliding Window, Sorting	Solution Using Sorting
438	Find All Anagrams in a String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String
567	Permutation in String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String, Two Pointers
713	Subarray Product Less Than K	Java	$\text{\color{Dandelion}Medium}$	Array, Sliding Window
904	Fruit Into Baskets	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sliding Window

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Sorting

#	Problem	Solution	Difficulty	Topics	Note
15	3Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Sorting, Two Pointers
16	3Sum Closest	Java	$\text{\color{Dandelion}Medium}$	Array, Sorting, Two Pointers
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
88	Merge Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Two Pointers
169	Majority Element	Java	$\text{\color{TealBlue}Easy}$	Array, Counting, Divide and Conquer, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm, Counting & Divide and Conquer or Counting & Hash Table
217	Contains Duplicate	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sorting	Solution Using Hash Map or Hash Set
220	Contains Duplicate III	Java	$\text{\color{Red}Hard}$	Array, Bucket Sort, Ordered Set, Sliding Window, Sorting	Solution Using Sliding Window & Bucket Sort or Sliding Window & Ordered Set
229	Majority Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting	Solution Using Boyer-Moore Majority Voting Algorithm or Counting & Hash Table
274	H-Index	Java with Counting Sort and Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Sorting
295	Find Median from Data Stream	Java with Heap (Priority Queue)	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers	Use of Sorting & Two Pointers resulted in TLE (Time Limit Exceed) for large datasets.
414	Third Maximum Number	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting	Solution without Sorting or Using Hash Table & Priority Queue
561	Array Partition	Java with Greedy & Counting Sort or Greedy & Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Greedy, Sorting
645	Set Mismatch	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting	Solution Using Bit Manipulation or Hash Table
905	Sort Array By Parity	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
922	Sort Array By Parity II	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
923	3Sum With Multiplicity	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting, Two Pointers	Solution Using Counting Hash Table
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
977	Squares of a Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Two Pointers
1051	Height Checker	Java with Counting Sort or Sorting	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Sorting
1122	Relative Sort Array	Java	$\text{\color{TealBlue}Easy}$	Array, Counting Sort, Hash Table, Sorting	Solution Using Hash Table
1337	The K Weakest Rows in a Matrix	Java with Sorting & Binary Search	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting	Solution Using Linear Search, Binary Search or Heap (Priority Queue) & Binary Search
1346	Check If N and Its Double Exist	Java with Binary Search & Sorting	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Hash Table, Sorting, Two Pointers	Solution Using Hash Table or Two Pointers
1636	Sort Array by Increasing Frequency	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Sorting
1657	Determine if Two Strings Are Close	Java with Sorting or Hash Table & Sorting	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting
1679	Max Number of K-Sum Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers	Solution Using Hash Table
1833	Maximum Ice Cream Bars	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting
1834	Single-Threaded CPU	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue), Sorting
2007	Find Original Array From Doubled Array	Java with Hash Table or Queue	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting	Solution Using Counting Array
2136	Earliest Possible Day of Full Bloom	Java	$\text{\color{Red}Hard}$	Array, Greedy, Sorting
2279	Maximum Bags With Full Capacity of Rocks	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting

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Stack

#	Problem	Solution	Difficulty	Topics	Note
94	Binary Tree Inorder Traversal	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Stack, Tree	Solution Using Depth-First Search (Recursive)
114	Flatten Binary Tree to Linked List	Java	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Linked List, Stack, Tree	Solution Using Depth-First Search (Recursive)
144	Binary Tree Preorder Traversal	Java	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree	Solution Using Depth-First Search (Recursive)
145	Binary Tree Postorder Traversal	Java	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree	Solution Using Depth-First Search (Recursive)
225	Implement Stack using Queues	Java	$\text{\color{TealBlue}Easy}$	Design, Queue, Stack
232	Implement Queue using Stacks	Java	$\text{\color{TealBlue}Easy}$	Design, Queue, Stack
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
503	Next Greater Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Monotonic Stack, Stack
1019	Next Greater Node In Linked List	Java	$\text{\color{Dandelion}Medium}$	Array, Linked List, Monotonic Stack, Stack
1047	Remove All Adjacent Duplicates In String	Java	$\text{\color{TealBlue}Easy}$	Stack, String	Solution Using StringBuilder only
1544	Make The String Great	Java	$\text{\color{TealBlue}Easy}$	Stack, String
2130	Maximum Twin Sum of a Linked List	Java with Two Pointers & Stack	$\text{\color{Dandelion}Medium}$	Linked List, Stack, Two Pointers	Solution Using only Two Pointers
2454	Next Greater Element IV	Java	$\text{\color{Red}Hard}$	Array, Monotonic Stack, Stack

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String

#	Problem	Solution	Difficulty	Topics	Note
3	Longest Substring Without Repeating Characters	Java with or without Hash Table	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String
5	Longest Palindromic Substring	Java with Dynamic Programming, Two Pointers with Approach 1 or Approach 2	$\text{\color{Dandelion}Medium}$	Dynamic Programming, String
6	Zigzag Conversion	Java	$\text{\color{Dandelion}Medium}$	String
12	Integer to Roman	Java with or without Hash Table	$\text{\color{Dandelion}Medium}$	Hash Table, Math, String
13	Roman to Integer	Java with or without Hash Table	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String
38	Count and Say	Java with Iterative or Recursive Algorithms	$\text{\color{Dandelion}Medium}$	String
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
151	Reverse Words in a String	Java with StringBuilder or Collections Implementations	$\text{\color{Dandelion}Medium}$	String, Two Pointers	Solution Using Two Pointers
205	Isomorphic Strings	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
257	Binary Tree Paths	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Backtracking, Binary Tree, Depth-First Search, String, Tree
290	Word Pattern	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
344	Reverse String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
345	Reverse Vowels of a String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
383	Ransom Note	Java with Counting Array or Hash Table	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String
412	Fizz Buzz	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation, String
438	Find All Anagrams in a String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String
520	Detect Capital	Java	$\text{\color{TealBlue}Easy}$	String
541	Reverse String II	Java with StringBuilder or Two Pointers	$\text{\color{TealBlue}Easy}$	String, Two Pointers
556	Next Greater Element III	Java	$\text{\color{Dandelion}Medium}$	Math, String, Two Pointers
557	Reverse Words in a String III	Java with String Builder or Two Pointers	$\text{\color{TealBlue}Easy}$	String, Two Pointers
567	Permutation in String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String, Two Pointers
609	Find Duplicate File in System	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, String
929	Unique Email Addresses	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
944	Delete Columns to Make Sorted	Java	$\text{\color{TealBlue}Easy}$	Array, String
953	Verifying an Alien Dictionary	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
988	Smallest String Starting From Leaf	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, String, Tree
1047	Remove All Adjacent Duplicates In String	Java	$\text{\color{TealBlue}Easy}$	Stack, String	Solution Using Stack
1061	Lexicographically Smallest Equivalent String	Java	$\text{\color{Dandelion}Medium}$	String, Union Find
1071	Greatest Common Divisor of Strings	Java	$\text{\color{TealBlue}Easy}$	Math, String
1154	Day of the Year	Java	$\text{\color{TealBlue}Easy}$	Math, String
1239	Maximum Length of a Concatenated String with Unique Characters	Java with Backtracking or Bit Manipulation	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String
1328	Break a Palindrome	Java	$\text{\color{Dandelion}Medium}$	Greedy, String
1446	Consecutive Characters	Java	$\text{\color{TealBlue}Easy}$	String
1544	Make The String Great	Java	$\text{\color{TealBlue}Easy}$	Stack, String
1578	Minimum Time to Make Rope Colorful	Java with Dynamic Programming or Greedy	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String
1592	Rearrange Spaces Between Words	Java	$\text{\color{TealBlue}Easy}$	String
1657	Determine if Two Strings Are Close	Java with Sorting or Hash Table & Sorting	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting
1662	Check If Two String Arrays are Equivalent	Java with or without Pointers	$\text{\color{TealBlue}Easy}$	Array, String
1704	Determine if String Halves Are Alike	Java	$\text{\color{TealBlue}Easy}$	Counting, String
1832	Check if the Sentence is Pangram	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String	Solution Using Bit Manipulation , Counting and Hash Table
1869	Longer Contiguous Segments of Ones than Zeros	Java	$\text{\color{TealBlue}Easy}$	String
1935	Maximum Number of Words You Can Type	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
2023	Number of Pairs of Strings With Concatenation Equal to Target	Java	$\text{\color{Dandelion}Medium}$	Array, String
2129	Capitalize the Title	Java	$\text{\color{TealBlue}Easy}$	String
2131	Longest Palindrome by Concatenating Two Letter Words	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String
2315	Count Asterisks	Java	$\text{\color{TealBlue}Easy}$	String
2351	First Letter to Appear Twice	Java with Counting or Hash Table	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String
2399	Check Distances Between Same Letters	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
2437	Number of Valid Clock Times	Java	$\text{\color{TealBlue}Easy}$	Enumeration, String
2451	Odd String Difference	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String
2452	Words Within Two Edits of Dictionary	Java	$\text{\color{Dandelion}Medium}$	Array, String

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Tree

#	Problem	Solution	Difficulty	Topics
94	Binary Tree Inorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Stack, Tree
98	Validate Binary Search Tree	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Search Tree, Binary Tree, Depth-First Search, Tree
100	Same Tree	Java with Breadth-First Search or Depth-First Search	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
102	Binary Tree Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
103	Binary Tree Zigzag Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
104	Maximum Depth of Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
107	Binary Tree Level Order Traversal II	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Breadth-First Search, Tree
110	Balanced Binary Tree	Java	$\text{\color{TealBlue}Easy}$	Binary Tree, Depth-First Search, Tree
111	Minimum Depth of Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
112	Path Sum	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
113	Path Sum II	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Binary Tree, Depth-First Search, Tree
114	Flatten Binary Tree to Linked List	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Linked List, Stack, Tree
129	Sum Root to Leaf Numbers	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
144	Binary Tree Preorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree
145	Binary Tree Postorder Traversal	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search, Depth-First Search, Stack, Tree
222	Count Complete Tree Nodes	Java	$\text{\color{Dandelion}Medium}$	Binary Search, Binary Tree, Depth-First Search, Tree
226	Invert Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
257	Binary Tree Paths	Java with Backtracking and Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Backtracking, Binary Tree, Depth-First Search, String, Tree
404	Sum of Left Leaves	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Tree, Breadth-First Search, Depth-First Search, Tree
429	N-ary Tree Level Order Traversal	Java	$\text{\color{Dandelion}Medium}$	Breadth-First Search, Tree
437	Path Sum III	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
559	Maximum Depth of Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Tree
653	Two Sum IV - Input is a BST	Java with Breadth-First Search (with Hash Table), Breadth-First Search (without Hash Table), Depth-First Search (Iterative), Depth-First Search (Recursive), Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers
783	Minimum Distance Between BST Nodes	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Tree
988	Smallest String Starting From Leaf	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, String, Tree
1026	Maximum Difference Between Node and Ancestor	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Depth-First Search, Tree
1457	Pseudo-Palindromic Paths in a Binary Tree	Java with Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Binary Tree, Bit Manipulation, Breadth-First Search, Depth-First Search, Tree

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Two Pointers

#	Problem	Solution	Difficulty	Topics	Note
15	3Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Sorting, Two Pointers
16	3Sum Closest	Java	$\text{\color{Dandelion}Medium}$	Array, Sorting, Two Pointers
19	Remove Nth Node From End of List	Java with One Pass or Two Passes	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers
26	Remove Duplicates from Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
27	Remove Element	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
80	Remove Duplicates from Sorted Array II	Java	$\text{\color{Dandelion}Medium}$	Array, Two Pointers
88	Merge Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Sorting
151	Reverse Words in a String	Java	$\text{\color{Dandelion}Medium}$	String, Two Pointers	Solution Using Collections or StringBuilder
167	Two Sum II - Input Array is Sorted	Java	$\text{\color{Dandelion}Medium}$	Array, Binary Search, Two Pointers	Solution Using Binary Search
283	Move Zeroes	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
295	Find Median from Data Stream	Java with Heap (Priority Queue)	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers	Use of Sorting & Two Pointers resulted in TLE (Time Limit Exceed) for large datasets.
344	Reverse String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
345	Reverse Vowels of a String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
541	Reverse String II	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers	Solution Using StringBuilder
556	Next Greater Element III	Java	$\text{\color{Dandelion}Medium}$	Math, String, Two Pointers
557	Reverse Words in a String III	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers	Solution Using String Builder
567	Permutation in String	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Sliding Window, String, Two Pointers
653	Two Sum IV - Input is a BST	Java with Two Pointers (with Iterative Inorder Traversal) or Two Pointers (with Recursive Inorder Traversal)	$\text{\color{TealBlue}Easy}$	Binary Search Tree, Binary Tree, Breadth-First Search, Depth-First Search, Hash Table, Tree, Two Pointers	Solution Using Breadth-First Search (with Hash Table), Breadth-First Search (without Hash Table), Depth-First Search (Iterative) or Depth-First Search (Recursive)
876	Middle of the Linked List	Java	$\text{\color{TealBlue}Easy}$	Linked List, Two Pointers
905	Sort Array By Parity	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
922	Sort Array By Parity II	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
923	3Sum With Multiplicity	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table, Sorting, Two Pointers	Solution Using Counting Hash Table
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
977	Squares of a Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Sorting
1089	Duplicate Zeros	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers	Solution Without Two Pointers
1346	Check If N and Its Double Exist	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Hash Table, Sorting, Two Pointers	Solution Using Binary Search & Sorting or Hash Table
1679	Max Number of K-Sum Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers	Solution Using Hash Table
2095	Delete the Middle Node of a Linked List	Java	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers
2130	Maximum Twin Sum of a Linked List	Java with Two Pointers or Two Pointers & Stack	$\text{\color{Dandelion}Medium}$	Linked List, Stack, Two Pointers

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Union Find

#	Problem	Solution	Difficulty	Topics	Note
130	Surrounded Regions	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Breadth-First Search or Depth-First Search
200	Number of Islands	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Breadth-First Search or Depth-First Search
1061	Lexicographically Smallest Equivalent String	Java	$\text{\color{Dandelion}Medium}$	String, Union Find
1971	Find if Path Exists in Graph	Java	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find	Solution Using Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)

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