# Question

Formatted question description: https://leetcode.ca/all/173.html

 173	Binary Search Tree Iterator

Implement an iterator over a binary search tree (BST).
Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

@tag-tree


# Algorithm

The non-recursive form of the in-order traversal of the binary tree requires an additional definition of a stack to assist. The building rule of the binary search tree is left<root<right, and the in-order traversal can extract all nodes from small to large.

Java