leetcode 098 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.

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/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool valid(TreeNode* node, TreeNode* &pre) {if(node==NULL) return true;if(valid(node->left, pre)) {if(pre!=NULL && pre->val >= node->val) return false;pre=node;return valid(node->right, pre);}return false;}bool isValidBST(TreeNode* root) {if(root == NULL) return true;TreeNode *pre=NULL;return valid(root, pre);}};