[leetcode: Python]225. Implement Stack using Queues

题目：
mplement 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).

方法一：性能45ms

class MyStack(object):    def __init__(self):        """        Initialize your data structure here.        """        self.myStack = []    def push(self, x):        """        Push element x onto stack.        :type x: int        :rtype: void        """        self.myStack.append(x)    def pop(self):        """        Removes the element on top of the stack and returns that element.        :rtype: int        """        if not self.empty():            return self.myStack.pop()    def top(self):        """        Get the top element.        :rtype: int        """        if not self.empty():            return self.myStack[-1]    def empty(self):        """        Returns whether the stack is empty.        :rtype: bool        """        return len(self.myStack) == 0# Your MyStack object will be instantiated and called as such:# obj = MyStack()# obj.push(x)# param_2 = obj.pop()# param_3 = obj.top()# param_4 = obj.empty()

方法二：性能32ms

class MyStack(object):    def __init__(self):        """        Initialize your data structure here.        """        self.queue = []    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        """        _queue = self.queue        for _ in range(len(_queue)-1):            _queue.append(_queue.pop(0))        return _queue.pop(0)    def top(self):        """        Get the top element.        :rtype: int        """        tmp = self.pop()        self.queue.append(tmp)        return tmp    def empty(self):        """        Returns whether the stack is empty.        :rtype: bool        """        return 0 == len(self.queue)# Your MyStack object will be instantiated and called as such:# obj = MyStack()# obj.push(x)# param_2 = obj.pop()# param_3 = obj.top()# param_4 = obj.empty()