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.

Example 1:

    2
   / \
  1   3

Binary tree

[2,1,3]

, return true.

Example 2:

    1
   / \
  2   3

Binary tree

[1,2,3]

, return false.

need to ask whether it is long or int data type

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }

        long min = Long.MIN_VALUE;
        long max = Long.MAX_VALUE;
        return helper(root, min, max );

    }

    private boolean helper(TreeNode root, long min, long max) {
        if (root == null) {
            return true;
        } 

        if (root.val <= min || root.val >= max) {
            return false;

        } else {
            return helper(root.left, min, root.val) && helper(root.right, root.val, max);
        }
    }
}