https://www.hackerrank.com/challenges/super-digit

Erlang - Medium

% Enter your code here. Read input from STDIN. Print output to STDOUT
% Your class should be named solution

-module(solution).
-export([main/0]).

main() ->
    {ok,[N,K]} = io:fread("","~s~d"),
    P  = ktimes(N,K,""),
    {Num,_} = string:to_integer(P),
    io:format("~p",[splitONum(Num)]).

%string:concat is ++ operator
ktimes(_,0,R)-> R;
ktimes(IntStr,K,R)-> ktimes(IntStr,K-1,string:concat(IntStr,R)) .

splitONum(N)when N < 10 -> N;
splitONum(N)-> splitONum(foo(N,0)).

%N denotes the number , S as SUM
foo(0,S)-> S;
foo(N,S) -> foo(N div 10,(N rem 10) + S).

-module(solution).
-export([main/0]).

main() ->
    {ok,[N,K]} = io:fread("","~d~d"),
    io:format("~w~n",[digitalRoot((N*K)rem 9)]).

digitalRoot(R) when R == 0 -> 9;
digitalRoot(R)  -> R.