[225] Implement Stack using Queues

1. 題目描述

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.
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).

使用隊列實現棧。只能使用隊列中的基本方法包括add()offer()/element()peek()/remove()poll()。

2. 解題思路

一開始覺得應該和[232] Implement Queue using Stacks差不多，使用兩個隊列翻轉一下就可以了，後來仔細一想，不對，棧因爲後進先出的特性，通過兩個棧就可以進行翻轉，使用兩個隊列進行一次操作是無法將隊列翻轉過來的。如[1,2,3]出隊到另一個隊列還是[1,2,3]。那麼就只能使用兩個隊列來回倒騰的方式了，隊列中其中一個隊列負責push，當進行一次pop時，將這個隊列中除最後一個元素的所有元素移動到另一個隊列中，剩下的最後一個pop，並且切換責任，使另一個隊列作爲接受push的主隊列。過程如下圖所示：

3. Code

import java.util.Queue;
import java.util.LinkedList;

class MyStack {
    Queue<Integer> queue1 = new LinkedList<>();
    Queue<Integer> queue2 = new LinkedList<>();
    int mainQueue = 1;

    // Push element x onto stack.
    public void push(int x) {
        if(mainQueue == 1){
            queue1.offer(x);
        }
        else
        {
            queue2.offer(x);
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        if(mainQueue == 1)
        {
            move12();
            mainQueue = 2;
            queue1.poll();
        }
        else
        {
            move21();
            mainQueue = 1;
            queue2.poll();
        }
    }

    // Get the top element.
    public int top() {
        if(mainQueue == 1)
        {
            move12();
            return queue1.peek();
        }
        else
        {
            move21();
            return queue2.peek();
        }
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue1.isEmpty() && queue2.isEmpty();
    }

    private void move12()
    {
        // 轉移queue1到queue2，保留一個元素
        while(queue1.size() > 1)
        {
            queue2.offer(queue1.poll());
        }
    }

    private void move21()
    {
        // 轉移queue2到queue1，保留一個元素
        while(queue2.size() > 1)
        {
            queue1.offer(queue2.poll());
        }
    }
}