題目，找出兩節點最小差值，思路以中序概念來想，經過 BST 樹經過中序遍歷後其結果會是排序的樣子，因此我們只要當前的值和左邊的值進行相減逐一比較即可獲得答案。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
int res = Integer.MAX_VALUE;
TreeNode node = null;
public int minDiffInBST(TreeNode root) {
if (root == null) return 0;
minDiffInBST(root.left);
if (node != null){
System.out.println(root.val +" "+ node.val);
res = Math.min(res, root.val - node.val);
}
node = root;
minDiffInBST(root.right);
return res;
}
}