נתונות פעולות רקורסיביות Mystery ו-Secret:

public static int Mystery(string s1, string s2)
{
    if (s1.Length == 0 && s2.Length == 0) return 0;
    if (s1.Length == 0) return -1;
    if (s2.Length == 0) return 1;
    if (s1[0] < s2[0]) return -1;
    if (s1[0] > s2[0]) return 1;
    return Mystery(s1.Substring(1), s2.Substring(1));
}

public static string[] Secret(string[] arr, string s)
{
    string[] temp = new string[arr.Length];
    SecretHelp(arr, temp, s, 0, 0, temp.Length - 1);
    return temp;
}

private static void SecretHelp(string[] arr, string[] temp, string s, int i, int left, int right)
{
    if (i < arr.Length)
    {
        if (Mystery(arr[i], s) < 0)
        {
            temp[left] = arr[i];
            SecretHelp(arr, temp, s, i + 1, left + 1, right);
        }
        else
        {
            temp[right] = arr[i];
            SecretHelp(arr, temp, s, i + 1, left, right - 1);
        }
    }
}