public class Queue
{
    QueueItem head;
    QueueItem tail;

    public void Enqueue(int value)
    {
        var item = new QueueItem { Value = value, Next = null };
        if (head == null)
        {
            tail = item;
            head = item;
        }
        else
        {
            tail.Next = item;
            tail = item;
        }
    }

    public int Dequeue()
    {
        if (head == null)
        {
            throw new InvalidOperationException();
        }
        var result = head.Value;
        head = head.Next;
        if (head == null)
        {
            tail = null;
        }
        return result;
    }
}