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.

Tips: Recursive做法，可以试着重做一遍。O(n)

Code:

public class Solution {
    int min = Integer.MAX_VALUE;
    Integer 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);
        }
        prev = root.val;

        getMinimumDifference(root.right);

        return min;
    }

}