题目：

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less
  than the node‘s key.


• The right subtree of a node contains only nodes with
  keys greater than the node‘s key.


• Both the left and right subtrees must also be binary search trees.

解题思路：

中序遍历二叉树，设置pre指针，在遍历过程中记录是否发生违反BST的节点的存在。

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

13     TreeNode * pre = NULL;

14     bool isValidBST(TreeNode *root) {

15         if (root == NULL) return true;

16

17         if (root->left != NULL && !isValidBST(root->left)) return false;

18         if (pre != NULL && pre->val >= root->val) return false;

19         pre = root;

20         if (root->right != NULL && !isValidBST(root->right)) return false;

21         return true;

22     }

23 };

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