Minimum Absolute Difference in BST

Input: root = [4, 2, 6, 1, 3]
Output: 1

      4
     / \
    2   6
   / \
  1   3

Explanation: The minimum absolute difference is 1 (between nodes 1 and 2, or 2 and 3, or 3 and 4).

Input: root = [1, 0, 48, null, null, 12, 49]
Output: 1

      1
     / \
    0  48
       / \
     12  49

Explanation: The minimum absolute difference is 1 (between nodes 0 and 1).

Input: root = [236, 104, 701, null, 227, null, 911]
Output: 9

Explanation: The sorted values are [104, 227, 236, 701, 911]. The minimum difference is 236 - 227 = 9.