// 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
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return helper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    public boolean helper(TreeNode node, int min, int max){
    	if(node == null) return true;

    	if(node.val <= min || node.val >= max){
    		return false;
    	}else{
    		return helper(node.left, min, node.val) && helper(node.right, node.val, max);
    	}
    }
}
add validate binary search tree solution