[225] Implement Stack using Queues

https://leetcode.com/problems/implement-stack-using-queues/description/

• algorithms
• Easy (36.69%)
• Source Code: 225.implement-stack-using-queues.py
• Total Accepted: 125.1K
• Total Submissions: 323.3K
• Testcase Example: '["MyStack","push","push","top","pop","empty"]\n[[],[1],[2],[],[],[]]'

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.

Example:

MyStack stack = new MyStack();

stack.push(1); stack.push(2);
stack.top(); // returns 2 stack.pop(); // returns 2 stack.empty(); // returns false

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).

# coding: utf-8
# 队列一定是先进先出的. right in, left out
class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue1 = []
        self.queue2 = []

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: None
        """
        self.queue1.append(x)


    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if not self.queue1:
            self.queue1, self.queue2 = self.queue2, self.queue1

        while len(self.queue1) > 1:
            self.queue2.append(self.queue1.pop(0))

        return self.queue1.pop(0)

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if not self.queue1:
            self.queue1, self.queue2 = self.queue2, self.queue1

        while len(self.queue1) > 1:
            self.queue2.append(self.queue1.pop(0))

        return self.queue1[0]

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return not self.queue1 and not self.queue2


# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# print obj.empty()
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()


# Input data:
# ["MyStack","push","push","top","pop","empty"]
# [[],[1],[2],[],[],[]]
#
# Actual
#   ? runtime: 20 ms
#   ? answer: [null,null,null,1,1,true]
#   ? stdout: ''
#
# Expected
#   ? runtime: 4 ms
#   ? answer: [null,null,null,2,2,false]
#   ? stdout: ''
#