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);
	}
};