題目：

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.

程式碼：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> list = new ArrayList<>();
    public void inorder(TreeNode root){
        if(root == null) return;
        inorder(root.left);
        list.add(root.val);
        inorder(root.right);
    }
    public int getMinimumDifference(TreeNode root) {
        inorder(root);
        int diff = Integer.MAX_VALUE;
        for(int i = 1; i < list.size(); i++){
            if(list.get(i) - list.get(i-1) < diff) diff = list.get(i) - list.get(i-1);
        }
        return diff;
    }
}