"""
A quack is a data structure combining properties of both stacks and queues. It can be viewed as a list of elements written left to right such that three operations are possible:

push(x): add a new item x to the left end of the list
pop(): remove and return the item on the left end of the list
pull(): remove the item on the right end of the list.
Implement a quack using three stacks and O(1) additional memory, so that the amortized time for any push, pop, or pull operation is O(1).
"""
