public static void Combinations(List<string> m, string output = "")
        {
            if (m.Count == 0)
            {
                Console.WriteLine(output);
                return;
            }
            for (int i = 0; i < m.Count; i++)
            {
                List<string> tmp = new List<string>(m);
                tmp.RemoveAt(i);
                Combinations(tmp, output + m[i]);
            }
        }