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：

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     // "fa" holds the last value that has been visited
     // "flag" is false when it(the given binary tree or its subtree) is an invalid BST
     void InOrder(TreeNode *root, int& fa, bool& flag) {
         if (root->left != NULL) {
             InOrder(root->left, fa, flag);
         }
         if (root->val <= fa)
             flag = false;
         fa = root->val;
         if (root->right != NULL) {
             InOrder(root->right, fa, flag);
         }
     }
     bool isValidBST(TreeNode *root) {
         if (root == NULL || root->left==NULL&&root->right==NULL) return true;
         // initialize "fa" as INT_MIN
         // I assume that there are no tree node's val equals to INT_MIN
         // and it does... (test case like this doesnt exist)
         int fa = INT_MIN;
         bool flag = true;
         InOrder(root, fa, flag);
         if (flag)
             return true;
         else
             return false;
     }
 };