Problem

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).

利用队列实现栈，这个题真是挺奇怪的。。。所以好像评价不高。。

Python3

# 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).

class MyStack:

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

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

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


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

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

分析

无话可说。。