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.

# Definition for a binary tree node.

# class TreeNode(object):

#     def __init__(self, x):

#         self.val = x

#         self.left = None

#         self.right = None

class Solution(object):

    def minDiffInBST(self, root):

        """

        :type root: TreeNode

        :rtype: int

        """

        diff=100000

        last_visited=None

        stack=[]

        node=root

        while node or stack:

            if node:

                stack.append(node)

                node=node.left

            else:

                node=stack.pop()

                if last_visited and abs(last_visited.val-node.val)<diff:

                    diff=abs(last_visited.val-node.val)

                last_visited=node

                node=node.right

        return diff