LeetCode ---　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     // "fa" holds the last value that has been visited
13     // "flag" is false when it(the given binary tree or its subtree) is an invalid BST
14     void InOrder(TreeNode *root, int& fa, bool& flag) {
15         if (root->left != NULL) {
16             InOrder(root->left, fa, flag);
17         }
18         if (root->val <= fa) 
19             flag = false;
20         fa = root->val;
21         if (root->right != NULL) {
22             InOrder(root->right, fa, flag);
23         }
24     }
25     bool isValidBST(TreeNode *root) {
26         if (root == NULL || root->left==NULL&&root->right==NULL) return true;
27         // initialize "fa" as INT_MIN
28         // I assume that there are no tree node‘s val equals to INT_MIN
29         // and it does... (test case like this doesnt exist)
30         int fa = INT_MIN;
31         bool flag = true;
32         InOrder(root, fa, flag);
33         if (flag)
34             return true;
35         else 
36             return false;
37     }
38 };