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.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

Solution:
Use value range. Return false if a root value is not within the range.

/**
 * 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 root, int min, int max){
        if(root == null) return true;
        if(root.val <= min || root.val >= max) return false;
        return (helper(root.left, min, root.val) && helper(root.right, root.val, max));
    }
}