java数据结构-链表实现队列

java数据结构-链表实现队列

删除与添加只对链表的头部操作复杂度是O(1)，如果对链表尾部操作复杂度为O(n)

队列需要在线性结构的一端插入元素在另一端删除删除元素，所以需要在两端同时操作，因此有一端的复杂度为O(n)

链表在头不管删除与插入比较方便，需要用head记录头元素，用tail记录尾节点。

尾巴tail插入元素，头head删除元素相对比较容易

public interface Queue<E> {
    int getSize();
    boolean isEmpty();
    void enqueue(E e);
    E dequeue();
    E getFront();
}

public class LinkedListQueue<E> implements Queue<E> {

    private class Node{
        public E e;
        public Node next;

        public Node(E e, Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e, null);
        }

        public Node(){
            this(null, null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    private Node head, tail;
    private int size;

    public LinkedListQueue(){
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public int getSize(){
        return size;
    }

    @Override
    public boolean isEmpty(){
        return size == 0;
    }

    @Override
    public void enqueue(E e){
        if(tail == null){
            tail = new Node(e);
            head = tail;
        }
        else{
            tail.next = new Node(e);
            tail = tail.next;
        }
        size ++;
    }

    @Override
    public E dequeue(){
        if(isEmpty())
            throw new IllegalArgumentException("Cannot dequeue from an empty queue.");

        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if(head == null)
            tail = null;
        size --;
        return retNode.e;
    }

    @Override
    public E getFront(){
        if(isEmpty())
            throw new IllegalArgumentException("Queue is empty.");
        return head.e;
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Queue: front ");

        Node cur = head;
        while(cur != null) {
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");
        return res.toString();
    }

    public static void main(String[] args){

        LinkedListQueue<Integer> queue = new LinkedListQueue<>();
        for(int i = 0 ; i < 10 ; i ++){
            queue.enqueue(i);
            System.out.println(queue);

            if(i % 3 == 2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}

性能比较

import java.util.Random;

public class Main {

    // 测试使用q运行opCount个enqueueu和dequeue操作所需要的时间，单位：秒
    private static double testQueue(Queue<Integer> q, int opCount){

        long startTime = System.nanoTime();

        Random random = new Random();
        for(int i = 0 ; i < opCount ; i ++)
            q.enqueue(random.nextInt(Integer.MAX_VALUE));
        for(int i = 0 ; i < opCount ; i ++)
            q.dequeue();

        long endTime = System.nanoTime();

        return (endTime - startTime) / 1000000000.0;
    }

    public static void main(String[] args) {

        int opCount = 100000;

        ArrayQueue<Integer> arrayQueue = new ArrayQueue<>();
        double time1 = testQueue(arrayQueue, opCount);
        System.out.println("ArrayQueue, time: " + time1 + " s");

        LoopQueue<Integer> loopQueue = new LoopQueue<>();
        double time2 = testQueue(loopQueue, opCount);
        System.out.println("LoopQueue, time: " + time2 + " s");

        LinkedListQueue<Integer> linkedListQueue = new LinkedListQueue<>();
        double time3 = testQueue(linkedListQueue, opCount);
        System.out.println("LinkedListQueue, time: " + time3 + " s");
    }
}