test.cs

        public static string Test2(string str) {
            if (str.Length == 0) return str;
            return str[str.Length - 1] + Test2(str.Substring(0, str.Length - 1));
        }

        public static string Test3(string str) {
            if (str.Length < 4) return str;
            return Test3(str.Substring(0, str.Length - 3)) + "," + str.Substring(str.Length - 3);
        }

        public static int? Test4(int[] vals, int index = 0, int? result = null) {
            if (vals.Length == 0 || index == vals.Length) return result;
            return Test4(vals, index + 1, result > vals[index] ? result.Value : vals[index]);
        }