defmodule ListUtilities do

  @doc """
  Flattens the given `list` of nested lists.
  ## Examples
      iex> ListUtilities.flatten([[1,2,[3]],4])
      [4, 3, 2, 1]
  """

  @spec flatten(list()) :: list()
  def flatten(list), do: do_flatten(list, [])

  defp do_flatten([head | tail], acc ) when is_list(head), do: do_flatten(tail, do_flatten(head, acc))
  defp do_flatten([head | tail], acc ), do: do_flatten(tail, [head| acc])
  defp do_flatten([], acc), do: acc

end

ListUtilities.flatten([[1,2,[3]],4]) |> IO.inspect