Leetcode:

 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.

注意min与max的作用

 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     bool isValidBST(TreeNode *root) {
13         // Start typing your C/C++ solution below
14         // DO NOT write int main() function
15         if (root == NULL)    return true;
16         return isValid(root, INT_MIN, INT_MAX);
17     }
18     bool isValid(TreeNode * root, int min, int max)
19     {
20         if (root == NULL)
21             return true;
22         if (root -> val <= min || root -> val >= max)
23             return false;
24         if (root -> left && root -> left -> val >= root -> val)
25             return false;
26         if (root -> right && root -> right -> val <= root -> val)
27             return false;
28         return isValid(root -> left, min, root -> val) && isValid(root -> right, root -> val, max);
29     }
30 };

 

转载于:https://www.cnblogs.com/caijinlong/archive/2013/05/02/3054155.html