package Queue;

public interface Queue<E> {
  //獲取隊(duì)列中元素個(gè)數(shù)
  int getSize();

  //隊(duì)列中元素是否為空
  boolean isEmpty();

  //入隊(duì)列
  void enqueue(E e);

  //出隊(duì)列
  public E dequeue();

  //獲取隊(duì)首元素
  public E getFront();
}

package Queue;

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

  //將Node節(jié)點(diǎn)設(shè)計(jì)成私有的類中類
  private class Node<E> {
    public E e;
    public Node next;

    //兩個(gè)參數(shù)的構(gòu)造函數(shù)
    public Node(E e, Node next) {
      this.e = e;
      this.next = next;
    }

    //一個(gè)參數(shù)的構(gòu)造函數(shù)
    public Node(E e) {
      this.e = e;
      this.next = null;
    }

    //無參構(gòu)造函數(shù)
    public Node() {
      this(null, null);
    }

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

  private Node<E> head, tail;

  private int size;


  //顯示初始化
  public LinkedListQueue() {
    head = null;
    tail = null;
    size = 0;
  }

  //獲取隊(duì)列中節(jié)點(diǎn)個(gè)數(shù)
  @Override
  public int getSize() {
    return size;
  }

  //隊(duì)列中是否為空
  @Override
  public boolean isEmpty() {
    return size == 0;
  }

  //鏈表尾部進(jìn)隊(duì)操作
  @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++;
  }

  //鏈表頭部出隊(duì)操作
  @Override
  public E dequeue() {
    if (isEmpty()) {
      throw new IllegalArgumentException("鏈表為空");
    }

    Node<E> retNode = head;
    head = head.next;
    retNode.next = null;
    if (head == null) {//當(dāng)鏈表只有一個(gè)元素時(shí)
      tail = null;
    }

    size--;
    return retNode.e;
  }

  //獲取隊(duì)首元素
  @Override
  public E getFront() {
    if (isEmpty()) {
      throw new IllegalArgumentException("鏈表為空");
    }
    return head.e;
  }


  //為了便于測試，重寫object類toString()方法
  @Override
  public String toString() {
    StringBuilder res = new StringBuilder();
    res.append("Queue: front ");
    Node<E> cur = head;
    while (cur != null) {
      res.append(cur + "->");
      cur = cur.next;
    }
    res.append("NULL tail");
    return res.toString();
  }


}

 //測試用例
  public static void main(String[] args) {
    LinkedListQueue<Integer> queue = new LinkedListQueue<Integer>();
    for (int i = 0; i < 10; i++) {
      queue.enqueue(i);
      System.out.println(queue);

      if (i % 3 == 2) {//每添加3個(gè)元素出隊(duì)列一個(gè)
        queue.dequeue();
        System.out.println(queue);
      }

    }

  }