[LeetCode] Validate Binary Search Tree

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            int pre=INT_MIN;return inorder(root,pre);    }bool inorder(TreeNode* root,int& pre){if (!root )return true;if (!inorder(root->left,pre))return false;if ( pre>=root->val)return false;pre=root->val;return inorder(root->right,pre);}};