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Given the root of a Binary Search Tree (BST), return the minimum absolute difference between the values of any two different nodes in the tree.
Example 1
Input: root = [4,2,6,1,3]
Output: 1
Explanation: An in-order walk gives the sorted values 1, 2, 3, 4, 6. The smallest gap between neighbours is 1 (for example 1 and 2).
Example 2
Input: root = [1,0,48,null,null,12,49]
Output: 1
Explanation: The sorted values are 0, 1, 12, 48, 49. Both 0/1 and 48/49 differ by 1, which is the minimum.
Example 3
Input: root = [5,3,7]
Output: 2
Explanation: The sorted values are 3, 5, 7. Neighbours differ by 2.
An in-order traversal of a BST visits values in sorted order, so the closest pair must be adjacent in that sequence. Track the previous value and take the smallest gap between consecutive visits.
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