public void AddLast(T data)
        {
            var 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)
        {
            var node = new DoublyNode <T>(data);
            var temp = head;

            node.Next = temp;
            head      = node;
            if (count == 0)
            {
                tail = head;
            }
            else
            {
                temp.Previous = node;
            }
            count++;
        }
        public T RemoveLast()
        {
            if (count == 0)
            {
                throw new InvalidOperationException();
            }
            T output = tail.Data;

            if (count == 1)
            {
                head = tail = null;
            }
            else
            {
                tail      = tail.Previous;
                tail.Next = null;
            }
            count--;
            return(output);
        }
        public T RemoveFirst()
        {
            if (count == 0)
            {
                throw new InvalidOperationException();
            }
            T output = head.Data;

            if (count == 1)
            {
                head = tail = null;
            }
            else
            {
                head          = head.Next;
                head.Previous = null;
            }
            count--;
            return(output);
        }
 public void Clear()
 {
     head  = null;
     tail  = null;
     count = 0;
 }