链表篇（3）—— 使用链表实现队列

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1 改进链表

• head 添加和删除元素很容易；
• tail 只是添加元素容易；
• head 作为 队首
• 没有 虚拟头结点，注意链表为空的情况；
  

2 基于链表的队列

• Queue.java

package linkedlist;

public interface Queue<E> {

    int getSize();
    boolean isEmpty();
    void enqueue(E e);
    E dequeue();
    E getFront();

}

• LinkedListQueue.java

package linkedlist;


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();
        }

        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();
        }

        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);
            }
        }
    }
}

Queue:front 0->Null tail
Queue:front 0->1->Null tail
Queue:front 0->1->2->Null tail
Queue:front 1->2->Null tail
Queue:front 1->2->3->Null tail
Queue:front 1->2->3->4->Null tail
Queue:front 1->2->3->4->5->Null tail
Queue:front 2->3->4->5->Null tail
Queue:front 2->3->4->5->6->Null tail
Queue:front 2->3->4->5->6->7->Null tail
Queue:front 2->3->4->5->6->7->8->Null tail
Queue:front 3->4->5->6->7->8->Null tail
Queue:front 3->4->5->6->7->8->9->Null tail

Process finished with exit code 0