# LeetCode题目：Validate Binary Search Tree

By | 2012 年 11 月 7 日

Validate Binary Search Tree
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.

```/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool isValid(TreeNode *root, int maxVal, int minVal, bool checkMax, bool checkMin) {
if(NULL == root) return true;
if(checkMax && root->val >= maxVal) return false;
if(checkMin && root->val <= minVal) return false;
bool leftValid = isValid(root->left, root->val, minVal, true, checkMin);
if(!leftValid) return false;
return isValid(root->right, maxVal, root->val, checkMax, true);
}
bool isValidBST(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
return isValid(root,0,0,false,false);
}
};
```