-module(recursion).
-export([fac/1, fac_tail/2]).

fac(N) -> if
    N =< 0 -> 1;
    N > 0 -> N * fac(N - 1)
end.

fac_tail(N, Acc) -> if
    N =< 0 -> Acc;
    N > 0 -> fac_tail(N - 1, Acc * N)
end.