[理解leetcode解法]98. Validate Binary Search Tree 判断是否二分查找树

98. Validate Binary Search Tree 判断是否二分查找树

#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.

#题解

/**  * 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 { private:     TreeNode* pre;     public:     bool isValidBST(TreeNode* root) {         if(root != NULL){             if(!isValidBST(root->left)){                 return false;             }             if(pre != NULL && root -> val <= pre -> val ) return false;   //1             pre=root;             if(!isValidBST(root->right)){                 return false;             }             return true;         }         return true;     } };

#题释

//1: 中序遍历