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.

TestBST.java

/**
 * 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        return helper(root, Integer.MAX_VALUE, Integer.MIN_VALUE);
    }
    
    public boolean helper(TreeNode root, int max, int min){
        if(root == null)
            return true;
        
        if(root.val < max && root.val > min && helper(root.left, root.val, min) &&
            helper(root.right, max, root.val))
            return true;
        
        return false; 
    }
}