验证二叉搜索树

 1 /**
 2  * Definition for a binary tree node.
 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     long last = LONG_MIN;
12 public:
13     bool isValidBST(TreeNode* root) {
14         if(root==nullptr)
15         {
16             return true;
17         }
18         
19         //比左树
20         if(!isValidBST(root->left))
21         {
22             return false;
23         }
24         if(last>=root->val)
25         {
26             return false;
27         }
28         
29         last =root->val;
30         //比右树
31         if(!isValidBST(root->right))
32         {
33             return false;
34         }
35         return true;
36         
37     }
38 };