public IEnumerable <T> BackEnumerator()
        {
            DoublyNode <T> cur = tail;

            while (cur != null)
            {
                yield return(cur.Data);

                cur = cur.Previous;
            }
        }
        IEnumerator <T> IEnumerable <T> .GetEnumerator()
        {
            DoublyNode <T> cur = head;

            while (cur != null)
            {
                yield return(cur.Data);

                cur = cur.Next;
            }
        }
        public bool Contains(T data)
        {
            DoublyNode <T> cur = head;

            while (cur != null)
            {
                if (cur.Data.Equals(data))
                {
                    return(true);
                }
                cur = cur.Next;
            }
            return(false);
        }
        public void Add(T data)
        {
            DoublyNode <T> node = new DoublyNode <T>(data);

            if (head == null)
            {
                head = node;
            }
            else
            {
                tail.Next     = node;
                node.Previous = tail;
            }
            tail = node;
            count++;
        }
        public void AddFirst(T data)
        {
            DoublyNode <T> node = new DoublyNode <T>(data);
            DoublyNode <T> temp = head;

            node.Next = temp;
            head      = node;
            if (count == 0)
            {
                tail = head;
            }
            else
            {
                temp.Previous = node;
            }
            count++;
        }
 public void Clear()
 {
     head  = null;
     tail  = null;
     count = 0;
 }