225.Implement Stack using Queues

问题描述：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).

python代码:

class MyStack(object):

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

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.stack.append(x)
        

    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if self.stack:
            return self.stack.pop()
        

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if self.stack:
            return self.stack[-1]
        

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return len(self.stack) == 0