[LeetCode] Validate Binary Search Tree

 1 class Solution {
 2     public:
 3         bool isValidBST(TreeNode *root) {
 4             
 5             if(root == NULL)
 6                 return true;
 7             vector<int> result;
 8             stack<TreeNode*> st;
 9 
10             TreeNode* p = root;
11 
12             // inorder traverse
13             while(p != NULL || st.size() != 0)
14             {
15                 while(p != NULL)
16                 {
17                     st.push(p);
18                     p = p->left;
19                 }
20 
21                 if(!st.empty())
22                 {
23                     p = st.top();
24                     st.pop();
25                     result.push_back(p->val);
26                     p = p->right;
27                 }
28             }
29             
30             //check if it is ascend
31             for(int i = 0; i < result.size()-1; i++)
32             {
33                 if(result[i] < result[i+1])
34                 ;
35                 else
36                     return false;
37             }
38             return true;
39 
40         }
41 };

 1 class Solution {
 2     public:
 3         bool isValidBST(TreeNode *root) {
 4             return isValidBST(root, INT_MIN, INT_MAX);
 5         }   
 6         bool isValidBST(TreeNode *root, int min, int max) 
 7         {   
 8             if(root == NULL) return true;
 9 
10             if( (root->val < max) && (root->val > min) &&
11                     isValidBST(root->left, min, root->val) &&
12                     isValidBST(root->right, root->val, max) 
13               )   
14                 return true;
15             else
16                 return false;
17         }   
18 };