20170630-leetcode-225 Implement Stack using Queues

1.Description

Implement the following operations of a stack using queues.
- push(x) – Push element x onto stack.
- pop() – Removes the element on top of the stack.
- top() – Get the top element.
- empty() – Return whether the stack is empty.
Notes:
You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
解读：用栈实现队列，在Python里面可以考虑使用deque

2.Solution

思路：使用双向队列dequeue
压栈：
1）使用另外的变量存储要压栈的数据，然后把原来的队列extend后面
2）只使用数据本身的队列，首先把压栈的数据append原来的队列，然后遍历前面n-1个数据，popleft同时append到这个队列中后面
弹出：双向队列popleft
top():取出双向队列的第一个元素

import collectionsclass MyStack(object):    def __init__(self):        """        Initialize your data structure here.        """        self._queue=collections.deque()    def push(self, x):        """        Push element x onto stack.        :type x: int        :rtype: void        """        q=self._queue        q.append(x)#把x加入到列表中        for _ in range(len(q)-1):#把要添加的元素之前            q.append(q.popleft())    def pop(self):        """        Removes the element on top of the stack and returns that element.        :rtype: int        """        return self._queue.popleft()    def top(self):        """        Get the top element.        :rtype: int        """        return self._queue[0]    def empty(self):        """        Returns whether the stack is empty.        :rtype: bool        """        return not len(self._queue)