LeetCode--Validate Binary Search Tree

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.

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool isValidBST(TreeNode *root) {if(root == NULL)return true;        vector<int> res;pre_search(root,res);for(int i=1; i<res.size(); i++){if(res[i] <= res[i-1])return false;}return true;    }void pre_search(TreeNode* root, vector<int>& res){if(root == NULL)return;pre_search(root->left,res);res.push_back(root->val);pre_search(root->right,res);}};