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.
Keep a stack and a current pointer, keep pushing node in stack when traversing into left tree
When poping from stack, output node value, and traverse the right subtree