superDigit(repeat k times of number_n) = superDigit( k * superDigit(number_n))

public static int superDigitHelper(String n) {
    int sum = 0;
    for(int i=0; i<n.length();i++){
      sum += Character.getNumericValue(n.charAt(i));
    }
    if(sum>=10){
        sum = superDigitHelper(String.valueOf(sum));
    }
    return sum;
}
public static int superDigit(String n, int k) {
    // Write your code here
    int superDigitN = superDigitHelper(n);
    int p = superDigitN*k;
    return superDigitHelper(String.valueOf(p));
}