队列：链式队列

基于链表实现的队列
时间复杂度：
入队:O(1)
出队:O(1)

接口类:

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

实现类

public class LinkedListQueue implements  Queue {
    //结点类
    class Node{
        public Node next;
        public E e ;

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

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

    }

    private Node head;//头结点
    private Node tail;//尾结点
    private int size;
    
    //初始化
    public LinkedListQueue(){
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public void enqueue(E e) {
        //队空
       if (tail == null){
           Node node = new Node(e);
           tail = node;
           head = tail;

       }
       else {
           Node node = new Node(e);
           tail.next = node;
           tail = tail.next;
       }
       size++;
    }

    @Override
    public E dequeue() {

        if (isEmpty()){
            throw new IllegalArgumentException("队列为空，无法删除元素");
        }

        Node reNode = head;
        head = head.next;
        reNode.next = null;
        //删除后队空了
        if (head == null){
            tail =null;
        }
        size--;
        return reNode.e;

    }

    @Override
    public E getFront() {
        if (isEmpty()){
            throw new IllegalArgumentException("队列为空");
        }

        return head.e;
    }

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

    @Override
    public boolean isEmpty() {
        return size == 0;
    }
    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("LinkedListQueue: front");
        Node cur = head;
        while (cur != null){
            res.append(cur.e + "->");
            cur = cur.next;
        }
        res.append("NULL  tail");
        return res.toString();
    }
}

测试：

public class Main {

    public static void main(String[] args) {
    Queue queue = new LinkedListQueue<>();

    for (int i = 0 ; i < 9 ; i ++){
        queue.enqueue(i);
        System.out.println(queue);
    }

    for (int i = 0 ; i < 5 ; i++)
         queue.dequeue();
    System.out.println(queue);
    for (int i = 0 ; i < 5 ; i ++){
            queue.enqueue(i);
    }
        System.out.println(queue);


      System.out.println(queue.getFront());       
      System.out.println(queue.getSize());
    }
}

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