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.

def superDigit(n, k): str1 = str(n) sum1 = 0 for char in str1: sum1 += int(char) sum1 = k*sum1 while True: str2 = str(sum1) sum1 = 0 for char in str2: sum1 += int(char) if sum1 < 10: break return sum1

Java 8 solution. Passed all cases. Make sure you are storing the sum of the digits as a Long.

    public static int superDigit(String n, int k) {
    // Write your code here

        if (n.length() == 1){
            return Integer.parseInt(n);
            
        }
        
        else{
            long sum = 0;
            for (int i = 0; i < n.length(); i++){
                sum += n.charAt(i) - '0';
            }
            sum = sum * k;
            return superDigit(Long.toString(sum), 1);
        }
        
    }

I guess test cases 0, 10 and 11 are not well designed, since they expect an output of 3 for an input of 148, an output of 8 for an input of 875, and an output of 9 for an input of 123. I am working with Swift.

php solution:

function superDigit($n, $k) {
    // Write your code here
    if(strlen($n) == 1 && $k <= 1)
        return $n;
        
    $sum = 0;
    
    for($i = 0; $i < strlen($n); $i++)
        $sum += (int) $n[$i] * $k;
    
    return superDigit((string)$sum, 1);
}