Implement Stack using Queues

from collections import deque

class QueueStack:
    def __init__(self):
        self.main = deque()    # the oldest item at the front, the newest at the back
        self.spare = deque()   # empty between operations

    def push(self, x):
        self.main.append(x)

    def _move_all_but_newest(self):
        # Pass every item except the newest to the spare queue, in order.
        while len(self.main) > 1:
            self.spare.append(self.main.popleft())

    def pop(self):
        self._move_all_but_newest()
        newest = self.main.popleft()
        self.main, self.spare = self.spare, self.main
        return newest

    def top(self):
        self._move_all_but_newest()
        newest = self.main.popleft()
        self.spare.append(newest)   # top keeps the item
        self.main, self.spare = self.spare, self.main
        return newest

    def empty(self):
        return not self.main

def stackOps(ops, args):
    stack = QueueStack()
    result = []
    for op, arg in zip(ops, args):
        if op == "push":
            stack.push(arg)
            result.append("null")
        elif op == "pop":
            result.append(str(stack.pop()))
        elif op == "top":
            result.append(str(stack.top()))
        else:
            result.append("true" if stack.empty() else "false")
    return result

from collections import deque

class QueueStack:
    def __init__(self):
        self.queue = deque()   # the newest item is always at the front

    def push(self, x):
        self.queue.append(x)
        # Rotate the older items behind x, so x comes out first.
        for _ in range(len(self.queue) - 1):
            self.queue.append(self.queue.popleft())

    def pop(self):
        return self.queue.popleft()

    def top(self):
        return self.queue[0]

    def empty(self):
        return not self.queue

def stackOps(ops, args):
    stack = QueueStack()
    result = []
    for op, arg in zip(ops, args):
        if op == "push":
            stack.push(arg)
            result.append("null")
        elif op == "pop":
            result.append(str(stack.pop()))
        elif op == "top":
            result.append(str(stack.top()))
        else:
            result.append("true" if stack.empty() else "false")
    return result