unsigned long digitSum(char *n, unsigned long len)
{
    unsigned long sum = 0 ;
    for (unsigned long i=0;i <len ; i++) {
        sum += n[i] -'0';
    }
    return sum ;
}

int getDigits(unsigned long long t)
{
    int d = 1;
    while(t>0) {
        t /= 10 ;
        d ++ ;
    }
    return d;
}

int superDigit(char* n, int k) {
    char * n2 = NULL ;
    unsigned long long sum = 0;
    sum = digitSum(n,strlen(n)) * k;
    int digits = getDigits(sum);
    
    printf("digits:%d\n",digits);
    
    n2 = malloc(digits+1);
    snprintf(n2,digits,"%llu",sum);
    n2[digits] = '\0';
    printf("n2:%s\n",n2);
    
    do {
        sum = digitSum(n2,strlen(n2));
        digits = getDigits(sum) ;
        //printf("n:%s, k:%d\n",n,k);      
        snprintf(n2,digits+1,"%llu",sum);
        n2[digits] ='\0';
        printf("sum:%llu digits:%d n:%s\n",sum,digits,n2);
    } while(sum>=10) ; 
    return sum ;
}