func SuperDigit(digits string, times int) string {
    if len(digits) <= 1 {
        return digits
    }
    return SuperDigit(digitSum(digits, times), 1)
}

func digitSum(digits string, times int) string {
    sum := 0
    for _, digit := range digits {
        sum += int(digit - '0')
    }
    return strconv.Itoa(sum * times)
}