三刷98. Validate Binary Search Tree

Medium
三刷还是没写出来，醉了。总的思路就是限定范围，这里用Integer type来定义范围，可以解决给的root的范围不知道的问题。

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