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 };