Python solution:

def superDigit(n: str, k: int) -> int:
    # sum digits of 'n' first and then multiply by k, 
    # instead of summing up digits of int(string * k):
    p = sum(list(map(int, list(n)))) * k
    while p > 10:     # same as len(str(p)) != 1
        p = sum(list(map(int, list(str(p)))))
        # can also use sum(int(digit) for digit in str(p))
    return p

Python 3 solution

Initially I tried expanding the string n by k multiple:

full_integer = n * k

This lead to runtime error in 4 hidden cases.

Instead, summing all digits in n then multiplying by k solves the runtime issue.

def int_to_int_arr(integer):
    return list(map(int, str(integer)))

def sum_array(n):
    if len(n) != 1:
        new_int = sum(n)
        int_arr = int_to_int_arr(new_int)
        return sum_array(int_arr)
    else:
        # One digit left in integer
        return n[0]

def superDigit(n, k):
    # Cast n string to int array
    int_arr = list(map(int, n))
    
    # Multiply sum by k
    sum_multiple = sum(int_arr) * k
    
    # Cast sum multiple to int array
    int_arr = int_to_int_arr(sum_multiple)
    return sum_array(int_arr)

Modern solution using java 8 streams:

    Long sum = 0L;        

    for (char digit : n.toCharArray()) {            
        sum += Character.getNumericValue(digit) * k;            
    }

    while(sum.toString().length() > 1) {

        List<String> remainingDigits= String.valueOf(sum)
                                            .chars()
                                            .mapToObj(c -> String.valueOf((char) c))
                                            .collect(Collectors.toList());

        sum = remainingDigits.stream().mapToLong(Long::parseLong).reduce(0L, Long::sum);

    }       

    return sum.intValue();   

}

Agree with others that the description isn't very clear in regards to what k is used for. Had to work it out from the failed test cases.

go is unfortunately pretty badly supported here.

My code produces a Runtime Error just for test cases 7,8,9, while the exact same code produces correct output on my machine (for case 7:the expected 7).

It's also suspicious that a Runtime Error happens only for a certain case...usually it's a real problem or not. :(( hackerrank these are things wanting to move me away to other platforms.