# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def minDiffInBST(self, root: Optional[TreeNode]) -> int:
        curr_smallest, prev = float("inf"), None
        
        def inorder(node):
            nonlocal curr_smallest, prev
            if node is None:
                return
            
            inorder(node.left)
            if prev is not None:
                curr_smallest = min(curr_smallest, node.val-prev.val)
            prev = node
            inorder(node.right)

        inorder(root)
        return curr_smallest