[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