如何使用两个堆栈实现一个队列? [英] How to implement a queue using two stacks?

问题描述

假设我们有两个堆栈并且没有其他临时变量.

Suppose we have two stacks and no other temporary variable.

是否可以仅使用两个堆栈构造"队列数据结构?

Is to possible to "construct" a queue data structure using only the two stacks?

推荐答案

保留 2 个堆栈，我们称它们为 inbox 和 outbox.

Keep 2 stacks, let's call them inbox and outbox.

入队:

• 将新元素推送到inbox

出队:

• 如果 outbox 为空，则从 inbox 弹出每个元素并将其推送到 outbox

• If outbox is empty, refill it by popping each element from inbox and pushing it onto outbox

从 outbox

使用此方法，每个元素将在每个堆栈中恰好出现一次 - 这意味着每个元素将被推送两次并弹出两次，从而提供摊销的常数时间操作.

Using this method, each element will be in each stack exactly once - meaning each element will be pushed twice and popped twice, giving amortized constant time operations.

这是一个 Java 实现:

Here's an implementation in Java:

public class Queue<E>
{

    private Stack<E> inbox = new Stack<E>();
    private Stack<E> outbox = new Stack<E>();

    public void queue(E item) {
        inbox.push(item);
    }

    public E dequeue() {
        if (outbox.isEmpty()) {
            while (!inbox.isEmpty()) {
               outbox.push(inbox.pop());
            }
        }
        return outbox.pop();
    }

}

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