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).

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

min = getMinimumDifference(root.right);

return min;
}
}