static String revAlternateK(String s,
for (int i = 2; i < s.Length;)
reverse(s.Substring(i, k).ToCharArray(), 0, k - 1) +
static String reverse(char []str, int start, int end)
return String.Join("", str);
public static void Main(String[] args)
String s = "abcdefghijk";
Console.WriteLine(revAlternateK(s, k, len));