Add verify-preorder-serialization-of-a-binary-tree based on stack

Author: beyondyyh

algorithms/binarysearch/69.mySqrt.go

@@ -8,7 +8,6 @@ func mySqrt(x int) int {
 // 每次mid取值为(l + r + 1) / 2
 // 这是为了防止当l = r - 1时，出现死循环的情况
 mid := (left + right + 1) >> 1
-// fmt.Printf("l:%d r:%d mid:%d\n", left, right, mid)
 if mid*mid <= x {
 left = mid
 } else {

algorithms/tree/331.isValidSerialization.go

@@ -1,8 +1,13 @@
 package tree
 
-import "strings"
+import (
+"strings"
 
-func isValidSerialization(preorder string) bool {
+"Leetcode/algorithms/kit"
+)
+
+// 读取到的结构只要符合二叉树性质而且不会在未读完之前就满足leaves = nodes + 1（完整的二叉树）即可
+func isValidSerialization1(preorder string) bool {
 var leaves, node int
 pres := strings.Split(preorder, ",")
 for i, s := range pres {
@@ -22,5 +27,35 @@ func isValidSerialization(preorder string) bool {
 if leaves == node+1 {
 return true
 }
+
 return false
 }
+
+// 基于stack
+// 使用栈来模拟二叉树的遍历，就是如果此时栈中的前两层都是null（"#"），那就将这颗子树弹出去，
+// 过程中要判断这棵子树的根节点是否为空，为空直接返回false，如果不为空将这棵树弹出去之后用一个"#"代替，
+// 这样一遍遍历下来栈中保留的就应该只有一个"#",对其进行判断，满足返回true，否则返回false
+func isValidSerialization2(preorder string) bool {
+if len(preorder) == 0 {
+return false
+}
+
+array := strings.Split(preorder, ",")
+stack := kit.NewStack()
+for _, ele := range array {
+stack.Push(ele)
+for stack.Len() >= 3 && stack.Peek().(string) == "#" && stack.Seek(stack.Len()-2).(string) == "#" {
+stack.Pop()
+stack.Pop()
+// 如果节点为空，返回false
+if stack.Peek().(string) == "#" {
+return false
+}
+stack.Pop()
+
+stack.Push("#")
+}
+}
+
+return stack.Len() == 1 && stack.Peek().(string) == "#"
+}

algorithms/tree/331.isValidSerialization_test.go

@@ -24,13 +24,17 @@ func Test_isValidSerialization(t *testing.T) {
 Input: "9,#,#,1",
 Expected: false,
 },
-// more test case
+// add more test cases
 }
 
 for _, tt := range cases {
 t.Run(tt.Name, func(t *testing.T) {
-if output := isValidSerialization(tt.Input.(string)); output != tt.Expected.(bool) {
-t.Errorf("isValidSerialization(%s)=%t, expected=%t", tt.Input, output, tt.Expected.(bool))
+if output := isValidSerialization1(tt.Input.(string)); output != tt.Expected.(bool) {
+t.Errorf("isValidSerialization1(%s)=%t, expected=%t", tt.Input, output, tt.Expected.(bool))
+}
+
+if output := isValidSerialization2(tt.Input.(string)); output != tt.Expected.(bool) {
+t.Errorf("isValidSerialization2(%s)=%t, expected=%t", tt.Input, output, tt.Expected.(bool))
 }
 })
 }