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.

Example 1:
2
/
1 3

Binary tree [2,1,3], return true.

Example 2:
1
/
2 3

Binary tree [1,2,3], return false.

解题之法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
bool isValidBST(TreeNode* root) {
    return isValidBST(root, NULL, NULL);
}

bool isValidBST(TreeNode* root, TreeNode* minNode, TreeNode* maxNode) {
    if(!root) return true;
    if(minNode && root->val <= minNode->val || maxNode && root->val >= maxNode->val)
        return false;
    return isValidBST(root->left, minNode, root) && isValidBST(root->right, root, maxNode);
}
};