225. Implement Stack using Queues 用队列实现栈

作者： 负雪明烛 id： fuxuemingzhu 个人博客： https://fuxuemingzhu.cn/

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题目地址：https://leetcode.com/problems/implement-stack-using-queues/#/description

题目描述

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:

1. 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.
2. 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.
3. You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题目大意

金庸队列的操作实现一个栈。

解题方法

一个队列就可以解决，不需要两个队列的。

每次新元素进队列的时候，先把当前的数字进入队列，然后把它前面的所有的元素翻转一下，翻到了它的后面。

Java代码如下：

public class MyStack {
    Queue<Integer> queue;
    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<Integer>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for(int i = 0; i < queue.size() -1; i++){
            queue.add(queue.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }

    /** Get the top element. */
    public int top() {
        return queue.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */

Python代码如下：

class MyStack(object):

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

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.que.append(x)
        for i in range(len(self.que) - 1):
            self.que.append(self.que.popleft())

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

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        return self.que[0]

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

# 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()

日期

2017 年 5 月 21 日 2018 年 11 月 24 日 —— 周六快乐