public static int superDigit(String n, int k) {
    long superdigit = Arrays.stream(n.split("")).mapToLong(Long::valueOf).sum()*k;
    while(superdigit>10) {
        superdigit = Arrays.stream(String.valueOf(superdigit).split("")).mapToLong(Long::valueOf).sum();
    }
    return (int)superdigit;
}