链表相比队列最大的不同依然在于链表的插入比较省时间，但是全局操作来看还是数组很省时间，因为链表的构建和连接代价比数组的创建要高很多。

由于队列不再是头插头删了，所以需要维护一个尾指针进行尾插头删

基本结构如下

	private Node head;	//head using for delete before
	private Node tail;	//tail using of insert after

	public void enqueue(String str) {
		Node newNode = new Node(str);

		if (isEmpty()) {
			head = newNode;
			tail = newNode;
		} else {
			tail.next = newNode;
			tail = newNode;
		}
	}

	public String dequeue() {
		String deItem;

		if (!isEmpty()) {
			deItem = head.item;
			head = head.next;
			return deItem;
		} else
			return "Queue is empty";
	}

	public boolean isEmpty() {
		return head == null;
	}

size

	public int size() {
		int size = 0;
		while (head != null) {
			head = head.next;
			size++;
		}
		return size;
	}