Implement Stack using Queues

这题是无法做到摊还O(1)的，要不pop浪费时间，要不push浪费时间，个人觉得什么双队列，单队列都没什么意义，下面是O(n)pop的算法

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue = collections.deque()
        self._queue = collections.deque()

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

    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        while len(self.queue) > 1:
            self._queue.appendleft(self.queue.pop())
        result = self.queue.pop()
        while self._queue:
            self.queue.appendleft(self._queue.pop())
        return result

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        while len(self.queue) > 1:
            self._queue.appendleft(self.queue.pop())
        return self.queue[0]

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

Implement Queue using Stacks

因为栈的特性，这里如果利用双栈实现queue的话虽然可能单个操作时间复杂度较高，但是摊还后还是O(1)的

class MyQueue(object):

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

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        self.stack.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        if self._stack:
            return self._stack.pop()
        while self.stack:
            self._stack.append(self.stack.pop())
        return self._stack.pop()

    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        if self._stack:
            return self._stack[-1]
        while self.stack:
            self._stack.append(self.stack.pop())
        return self._stack[-1]

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


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()