[leetcode]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 isValidBST(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(!root) return true;                if(!isValidBST(root -> left)) return false;        if(!isValidBST(root -> right)) return false;                if(root->left){             TreeNode *p = root->left;            while(p->right) p = p->right;            if(p->val >= root->val) return false;        }        if(root->right){            TreeNode *q = root->right;            while(q->left) q = q->left;            if(q->val <= root->val) return false;        }                return true;            }};