The minimum absolute difference [LeetCode] binary search tree

Given a non-negative all nodes of the binary search tree, the absolute value of the minimum difference between two arbitrary nodes in the tree request.

Example:

Input:

   1
    \
     3
    /
   2

Output:
1

Explanation: The
minimum absolute difference of 1, wherein the absolute value of difference of 1 and 2 is 1 (or 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 num = Integer.MAX_VALUE;
    TreeNode pre = null;
    public int getMinimumDifference(TreeNode root) {
        inOrder(root);
        return num;
    }
    public void inOrder(TreeNode root){
        if(root == null){
            return ;
        }
        inOrder(root.left);
        if(pre!=null){
            num = Math.min(num, root.val - pre.val);
        }
        pre = root;
        inOrder(root.right);
    }
}