queue.py

# Implement a queue with 2 stacks . Your queue should have an enqueue and a
# dequeue function and it should be "first in first out" (FIFO).
#
# Optimize for the time cost of mmm function calls on your queue. These can be
# any mix of enqueue and dequeue calls.
#
# Assume you already have a stack implementation and it gives O(1)
# time push and pop.

class Stack(list):
    def push(self, obj):
        self.append(obj)

    def pop(self):
        return super().pop()

class Queue():
    def __init__(self):
        self._stack0 = Stack()
        self._stack1 = Stack()

    def enqueue(self, obj):
        self._stack0.push(obj)

    def dequeue(self):
        if len(self._stack1) == 0:
            while len(self._stack0) > 0:
                self._stack1.push(self._stack0.pop())
        return self._stack1.pop()

if __name__ == '__main__':
    q = Queue()
    q.enqueue(1)
    q.enqueue(2)
    assert q.dequeue() == 1
    q.enqueue(3)
    q.enqueue(4)
    assert q.dequeue() == 2
    assert q.dequeue() == 3
    assert q.dequeue() == 4
    q.enqueue(3)
    q.enqueue(4)
    assert q.dequeue() == 3
    assert q.dequeue() == 4