Validate Binary Search Tree

 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     bool isValidBST(TreeNode *root) {
13         if( !root ) return true;
14         stack<TreeNode*> st;
15         TreeNode* cur = root;
16         while( cur ) {
17             st.push(cur);
18             cur = cur->left;
19         }
20         TreeNode* pre = 0;
21         while( !st.empty() ) {
22             cur = st.top();
23             st.pop();
24             if( pre && pre->val >= cur->val ) return false;
25             pre = cur;
26             cur = cur->right;
27             while( cur ) {
28                 st.push(cur);
29                 cur = cur->left;
30             }
31         }
32         return true;
33     }
34 };