LeetCode:Validate Binary Search Tree(iupdate)

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.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

中序遍历验证是否是..

/**
 * 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 {
    //中序遍历就可以
private:
    TreeNode *prenode;
public:
    bool isValidBST(TreeNode* root) {
        
        if(root)
        {
        if(!isValidBST(root->left)) return false;
        if(prenode&&prenode->val>=root->val)
            return false;
        prenode=root;
        
        if(!isValidBST(root->right)) return false;
        }
        return true;
        
    }
    
   
};