This answer is so awesome!!

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

LeetCode 98: Valid Binary Search Tree