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.

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1  / \ 2   3    /   4    \     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

Solution:

/** * 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 traverse(TreeNode* root, int &pre, bool &flag)    {        if(root == NULL) return true;        bool left = traverse(root->left, pre, flag);        if(flag && pre >= root->val) return false;        else        {            flag = true;            pre = root->val;        }        bool right = traverse(root->right, pre, flag);        return left && right;    }    bool isValidBST(TreeNode* root) {        int pre;        bool flag = false;        return traverse(root, pre, flag);    }};