［LeetCode］225. Implement Stack using Queues

225. Implement Stack using Queues

思路：用两个队列实现

import collections
class MyStack(object):
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue = collections.deque()
        self.queue_temp = collections.deque()
    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.queue.append(x)
    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if not self.queue:
            return None
        i, l = 0, len(self.queue)
        while i < l - 1:
            self.queue_temp.append(self.queue.popleft())
            i += 1
        val = self.queue.popleft()
        while self.queue_temp:
            self.queue.append(self.queue_temp.popleft())
        return val
    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if not self.queue:
            return None
        val = None
        while self.queue:
            val = self.queue.popleft()
            self.queue_temp.append(val)
        while self.queue_temp:
            self.queue.append(self.queue_temp.popleft())
        return val
    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return bool(len(self.queue) == 0)