Leetcode:
Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
- The left subtree of a node contains only nodes with keys less than the node's key.
- The right subtree of a node contains only nodes with keys greater than the node's key.
- Both the left and right subtrees must also be binary search trees.
注意min与max的作用
1 /**
2 * Definition for binary tree
3 * struct TreeNode {
4 * int val;
5 * TreeNode *left;
6 * TreeNode *right;
7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {}
8 * };
9 */
10 class Solution {
11 public:
12 bool isValidBST(TreeNode *root) {
13 // Start typing your C/C++ solution below
14 // DO NOT write int main() function
15 if (root == NULL) return true;
16 return isValid(root, INT_MIN, INT_MAX);
17 }
18 bool isValid(TreeNode * root, int min, int max)
19 {
20 if (root == NULL)
21 return true;
22 if (root -> val <= min || root -> val >= max)
23 return false;
24 if (root -> left && root -> left -> val >= root -> val)
25 return false;
26 if (root -> right && root -> right -> val <= root -> val)
27 return false;
28 return isValid(root -> left, min, root -> val) && isValid(root -> right, root -> val, max);
29 }
30 };
