Minimum Distance Between BST Nodes

Given a Binary Search Tree (BST) with the root node root, return the minimum difference between the values of any two different nodes in the tree.

Example :

Input: root = [4,2,6,1,3,null,null]
Output: 1
Explanation:
Note that root is a TreeNode object, not an array.

The given tree [4,2,6,1,3,null,null] is represented by the following diagram:

          4
        /   \
      2      6
     / \    
    1   3  

while the minimum difference in this tree is 1, it occurs between node 1 and node 2, also between node 3 and node 2.

Note:

1. The size of the BST will be between 2 and 100.

2. The BST is always valid, each node's value is an integer, and each node's value is different.

3. This question is the same as 530: https://leetcode.com/problems/minimum-absolute-difference-in-bst/

class Solution {
    Integer res = Integer.MAX_VALUE, pre = null;

    public int minDiffInBST(TreeNode root) {
        if (root.left != null)
            minDiffInBST(root.left);
        if (pre != null)
            res = Math.min(res, root.val - pre);
        pre = root.val;
        if (root.right != null)
            minDiffInBST(root.right);
        return res;
    }
}