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.
This question is the same as 783: https://leetcode.com/problems/minimum-distance-between-bst-nodes/

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int res = Integer.MAX_VALUE;
    TreeNode prev = null;
    
    public int getMinimumDifference(TreeNode root) {
        in(root);
        return res;
    }
    
    private void in(TreeNode root) {
        if (root == null) return;
        in(root.left);
        if (prev != null) {
            res = Math.min(res, Math.abs(root.val - prev.val));
        }
        prev = root;
        in(root.right);
    }
}