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.

 

/**

 * 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) 

    {

        return check(root,false,0,false,0);

    }

    bool check(TreeNode* root,bool hasmax,int max,bool hasmin,int min)

    {

        if(root==NULL) return true;

        if(hasmax && root->val>max)  return false;

        if(hasmin && root->val<min)  return false;

        return check(root->left,true,root->val-1,hasmin,min)

            && check(root->right,hasmax,max,true,root->val+1);

    }

};