0225._implement_stack_using_queues.md

225. Implement Stack using Queues

难度: Easy

刷题内容

原题连接

• https://leetcode.com/problems/implement-stack-using-queues/

内容描述

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.
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).

解题方案

思路 1 ******- 时间复杂度: Push O(N) + Pop O(1) - 空间复杂度: O(N)

class MyStack:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue = collections.deque()
        
    def push(self, x: int) -> None:
        """
        Push element x onto stack.
        """
        self.queue.append(x)
        for _ in range(len(self.queue) - 1):
            self.queue.append(self.queue.popleft())
        
    def pop(self) -> int:
        """
        Removes the element on top of the stack and returns that element.
        """
        return self.queue.popleft()

    def top(self) -> int:
        """
        Get the top element.
        """
        return self.queue[0]
        
    def empty(self) -> bool:
        """
        Returns whether the stack is empty.
        """
        return not len(self.queue)