[leetcode]98. Validate Binary Search Tree验证BST

Input:
    2
   /   1   3
Output: true

 1 class Solution {
 2     public boolean isValidBST(TreeNode root) {
 3         return helper(root, null, null);
 4     }
 5     
 6     boolean helper(TreeNode root, Integer min, Integer max) {
 7         if (root == null)
 8             return true;
 9         
10         if ((min != null && root.val <= min) || (max != null && root.val >= max))
11             return false;
12         
13         return helper(root.left, min, root.val) && helper(root.right, root.val, max);
14     }
15 }