root of a Binary Search Tree (BST), return the minimum difference between the values of any two different nodes in the tree.
Input: root = [4,2,6,1,3] Output: 1
Input: root = [1,0,48,null,null,12,49] Output: 1
0 <= Node.val <= 105
Note: This question is the same as 530: https://leetcode.com/problems/minimum-absolute-difference-in-bst/