Minimum Absolute Difference in BST

Given a binary search tree with non-negative values, find the minimum absolute difference between values of any two nodes.

Example:

Input:

   1
    
     3
    /
   2

Output:
1

Explanation:
The minimum absolute difference is 1, which is the difference between 2 and 1 (or between 2 and 3).

Note: There are at least two nodes in this BST.

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     int min = Integer.MAX_VALUE;
12     Integer prev = null;
13     
14     public int getMinimumDifference(TreeNode root) {
15         if (root == null) return min;
16         
17         getMinimumDifference(root.left);
18         
19         if (prev != null) {
20             min = Math.min(min, root.val - prev);
21         }
22         prev = root.val;
23         
24         getMinimumDifference(root.right);
25         
26         return min;
27     }
28     
29 }