队列

定义

• 队列是一种基于先进先出（FIFO）的集合类型

Java

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• 源码

import java.util.Iterator;

public class Queue<Item> implements Iterable<Item> {
    private Node first;// 指向最早添加的结点的链接
    private Node last;// 指向最近添加的结点的链接
    private int N;// 队列中的元素数量

    public boolean isEmpty() {
        return null == first;// 或：0 == N
    }

    public int size() {
        return N;
    }

    // 向表尾添加元素
    public void enqueue(Item item) {
        Node oldlast = last;
        last = new Node();
        last.item = item;
        last.next = null;
        if (isEmpty()) first = last;
        else oldlast.next = last;
        N++;
    }

    // 从表头删除元素
    public Item dequeue() {
        Item item = first.item;
        first = first.next;
        if (isEmpty()) last = null;
        N--;
        return item;
    }

    public Iterator<Item> iterator() {
        return new ListIterator();
    }

    private class Node {
        // 定义了结点的嵌套类
        Item item;
        Node next;
    }

    private class ListIterator implements Iterator<Item> {
        private Node current = first;

        public boolean hasNext() {
            return null != current;
        }

        public void remove() {
        }

        public Item next() {
            Item item = current.item;
            current = current.next;
            return item;
        }
    }
}

• 测试用例

public class Test {
    public static void main(String[] args) {

        String[] ss = {"to", "be", "or", "not", "to", "-", "be", "-", "-", "that", "-", "-", "-", "is"};
        Queue<String> q = new Queue<String>();
        for (String s : ss) {
            if (!s.equals("-")) {
                q.enqueue(s);
            } else {
                System.out.print(q.dequeue() + " ");
            }
        }
        System.out.println("(" + q.size() + " left on queue)");
    }
}

• 结果

to be or not to be (2 left on queue)