[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.

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}".

思考：记录parent有点繁，记录上下界。

 

/**

 * 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 DFS(TreeNode *root,int min,int max)

    {

        if(root)

        {

            if(min<root->val&&root->val<max)

            {

                return DFS(root->left,min,root->val)&&DFS(root->right,root->val,max);

            }

            else return false;

        }

        else return true;

    }

    bool isValidBST(TreeNode *root) {

        return DFS(root,INT_MIN,INT_MAX);

    }

};