defmodule Solution do
  @spec two_sum(nums :: [integer], target :: integer) :: [integer]
  def two_sum(nums, target) do
    process_num(nums, target)
  end

  @spec process_num(
          nums :: [integer],
          target :: integer,
          head_idx :: integer,
          indexes :: %{integer => integer}
        ) :: [integer]
  defp process_num([head | tail], target, head_idx \\ 0, indexes \\ %{}) do
    first_idx = indexes[target - head]

    if first_idx != nil do
      [first_idx, head_idx]
    else
      indexes = Map.put(indexes, head, head_idx)
      process_num(tail, target, head_idx + 1, indexes)
    end
  end
end