【LEETCODE】225-Implement Stack using Queues

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).

仅仅使用队列的标准操作即可——亦即只有如下操作是有效的：push to back（加入队尾），pop from front（弹出队首），size（取队列大小）以及is empty（判断是否为空）

就是只能用这几个操作，不能再用其他现成的操作

class Stack(object):
    def __init__(self):
        """
        initialize your data structure here.
        """
        self.queue=[]

    def push(self, x):
        """
        :type x: int
        :rtype: nothing
        """
        self.queue.append(x)

    def pop(self):
        """
        :rtype: nothing
        """
        for x in range(len(self.queue)-1):
            self.queue.append(self.queue.pop())
        self.queue.pop()

    def top(self):
        """
        :rtype: int
        """
        top=None
        for x in range(len(self.queue)):
            top=self.queue.pop()
            self.queue.append(top)
        return top
            

    def empty(self):
        """
        :rtype: bool
        """
        return len(self.queue)==0