public static string FormatRolls(List<int> rolls){
    string output = "";

    for (int i = 0, box = 1; i < rolls.Count; i++) {
        if (box >= 19 && rolls [i] == 10) {                                             //Strike in last frame
            output += "X";
            box++;
        } else if ((box > 1 && rolls [i] == 10 && rolls [i - 1] != 0) ||                //Strike in frames 1-9
                  (box == 1 && rolls [i] == 10)) {
            output += "X ";                                                             //Increment 2 since second
            box += 2;                                                                   //bowl in frame isn't needed
        } else if (rolls [i] == 0) {                                                    //Gutterball
            output += "-";
            box++;
        } else if (((box % 2) == 0 || box == 21) && rolls [i - 1] + rolls [i] == 10) {  //Spare
            output += "/";
            box++;
        } else {
            output += rolls [i].ToString ();                                            //1 to 9 pin falls
            box++;
        }
    }
    return output;
}