listsum

listsum.erl

-module(listsum).
-export([add_two_numbers/2]).

add_two_numbers(L1, L2) ->
    lists:reverse(add_two_lists(L1, L2, 0, [])).

add_two_lists([], [], 0, Acc) ->
    Acc;
add_two_lists([], [], Over, Acc) ->
    [Over|Acc];
add_two_lists([] = L1, [V2|L2], Over, Acc) ->
    next_step(L1, L2, V2 + Over, Acc);
add_two_lists([V1|L1], [] = L2, Over, Acc) ->
    next_step(L1, L2, V1 + Over, Acc);
add_two_lists([V1|L1], [V2|L2], Over, Acc) ->
    next_step(L1, L2, V1 + V2 + Over, Acc).

next_step(L1, L2, Sum, Acc) when Sum >= 10 ->
    add_two_lists(L1, L2, 1, [Sum - 10 | Acc]);
next_step(L1, L2, Sum, Acc) ->
    add_two_lists(L1, L2, 0, [Sum | Acc]).