Validate Binary Search Tree

一、问题描述

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.

二、思路

按照二叉树的定义，对一个节点来说，左子树所有值都小于它，右子树所有值都大于它。

三、代码

/** * 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){        TreeNode* pre = NULL;        return isValid(root,pre);    }    bool isValid(TreeNode* root,TreeNode* &prev){        if(root == NULL) return true;        if (!isValid(root->left, prev)) return false;        if (prev != NULL && prev->val >= root->val) return false;        prev = root;        return isValid(root -> right, prev);    }};