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.

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/**
 * 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 check(TreeNode *node,int lv,int rv){
		if(node==NULL) return true;
		return lv<node->val&&node->val<rv&&check(node->left,lv,node->val)&&check(node->right,node->val,rv);
	}
    bool isValidBST(TreeNode *root) {
    	return check(root,INT_MIN,INT_MAX);    
    }
};