1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12 
13     TreeNode * pre = NULL;
14     bool isValidBST(TreeNode *root) {
15         if (root == NULL) return true;
16 
17         if (root->left != NULL && !isValidBST(root->left)) return false;
18         if (pre != NULL && pre->val >= root->val) return false;
19         pre = root;
20         if (root->right != NULL && !isValidBST(root->right)) return false;
21         return true;
22     }
23 };