gakkou.erl

% this program solve the question from https://twitter.com/nya3_neko2/status/1481503969965142016

-module(gakkou).
-export([permutation/1, start/0, test/0]).

permutation(L) ->
    lists:reverse(permutation([], [], L)).

permutation(Results, Current, []) ->
    [lists:reverse(Current) | Results];
permutation(Results, Current, Rest) ->
    lists:foldl(fun(Elem, Res) ->
        permutation(Res, [Elem | Current], lists:delete(Elem, Rest)) end,
        Results, Rest).

start() ->
    lists:nth(100, lists:usort(permutation("GAKKOU"))).

test() ->
    io:format("~w~n", [[[]] =:= permutation([])]),
    io:format("~w~n", [["AB", "BA"] =:= permutation("AB")]).