LeetCode: Minimum Absolute Difference in Binary Search Tree

/**
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.
*/

public class MinimumAbsoluteDifferenceinBST{
int min = Integer.MAX_VALUE;
    TreeNode prev = null;

    public int getMinimumDifference(TreeNode root) {
        if (root == null) return min;

        getMinimumDifference(root.left);

        if (prev != null) {
            min = Math.min(min, root.val - prev.val);
        }
        prev = root;

        getMinimumDifference(root.right);

        return min;
    }
}