LeetCode --- Validate Binary Search Tree

题目链接

判断一颗二叉树是否是二叉搜索树（二叉排序树），也就是BST

如果该二叉树是BST,　那么对其中序遍历，所得序列一定是单调递增的（不考虑有重复数值的情况）

附上代码：

 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 };