public static int superDigit(String n, int k) {
        long sum = 0;
        for (char digit : n.toCharArray()) {
            sum += Character.getNumericValue(digit);
        }

        sum *= k;
        return superDigitHelper(sum);
    }

    private static int superDigitHelper(long n) {
        if (n < 10) return (int) n;

        long nextSum = 0;
        while (n > 0) {
            nextSum += n % 10;
            n /= 10;
        }

        return superDigitHelper(nextSum);
    }