Implement the following operations of a queue using stacks.

• push(x) – Push element x to the back of queue.
• pop() – Removes the element from in front of queue.
• peek() – Get the front element.
• empty() – Return whether the queue is empty.

  Notes:

• You must use only standard operations of a stack – which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Solution

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class Queue(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
        """
        self.queue.pop(0)

    def peek(self):
        """
        :rtype: int
        """
        # peek 的元素位queue[0]
        return self.queue[0]

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