update: 18

Author: hijiangtao

README.md

@@ -22,6 +22,7 @@ This is the solutions collection of my LeetCode submissions, most of them are pr
 | 15 | [3Sum](https://leetcode.com/problems/3sum/) | [JavaScript](./src/3sum/res.js) | Medium |
 | 16 | [3Sum Closest](https://leetcode.com/problems/3sum-closest/) | [JavaScript](./src/3sum-closest/res.js) | Medium |
 | 17 | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/) | [JavaScript](./src/letter-combinations-of-a-phone-number/res.js) | Medium |
+| 18 | [4sum](https://leetcode.com/problems/4sum/) | [TypeScript](./src/4sum/res.ts) | Medium |
 | 19 | [Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/) | [JavaScript](./src/remove-nth-node-from-end-of-list/res.js) | Medium |
 | 20 | [valid-parentheses](https://leetcode.com/problems/valid-parentheses/) | [TypeScript](./src/valid-parentheses/res.ts) | Easy |
 | 22 | [Generate Parentheses](https://leetcode.com/problems/generate-parentheses/) | [JavaScript](./src/generate-parentheses/res.js) | Medium |

src/4sum/res.ts

@@ -0,0 +1,44 @@
+function fourSum(nums: number[], target: number): number[][] {
+const res: number[][] = [];
+
+if (nums.length < 4) {
+return res;
+}
+
+nums.sort((a, b) => a - b);
+
+for (let i = 0; i < nums.length-3; i++) {
+if (i-1 >=0 && nums[i] === nums[i-1]) {
+continue;
+}
+for (let j = i+1; j < nums.length-2; j++) {
+if (j > i+1 && nums[j] === nums[j-1]) {
+continue;
+}
+
+let k = j+1, l = nums.length-1;
+while (k < l) {
+const sum = nums[i] + nums[j] + nums[k] + nums[l];
+
+if (sum === target) {
+res.push([nums[i], nums[j], nums[k], nums[l]]);
+
+while (k < l && nums[k] === nums[k+1]) {
+k++;
+}
+while (k < l && nums[l] === nums[l-1]) {
+l--;
+}
+k++;
+l--;
+} else if (sum < target) {
+k++;
+} else if (sum > target) {
+l--;
+}
+}
+}
+}
+
+return res;
+};