带尾结点的链表实现队列

• 头结点删除元素，尾结点插入元素
• 由于没有dummyHead, 要注意链表为空的情况

package com.hoho.datastruture.queue;
import com.hoho.datastruture.LinkedList.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() {
        this.head = null;
        this.tail = null;
        this.size = 0;
    }
    @Override
    public void enqueue(E e) {
        //尾部进队列
        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("队列为空，无法出队");
        }
        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("队列为空，无法出队");
        }
        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("Queue: front ");
        Node current = head;
        while (current != null) {
            res.append(current + " -> ");
            current = current.next;
        }
        res.append("NULL tail");
        return res.toString();
    }
}