public int? this[int start, int end]
        {
            get
            {
                Node tmp = head;
                int? min = null;
                int begin = 0;
                while (tmp != null && end >= begin)
                {
                    if (begin++ >= start)
                    {
                        min = min ?? tmp.data;
                        min = min > tmp.data ? tmp.data : min;
                    }
                    tmp = tmp.next;
                }
                return min;
            }
        }