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);
    }
};