defmodule Leetcode do
  @doc """
    LeetCode #1: two_sum

    TODO: store a map of complements (differences) as iterating for O(n)
  """

  @spec two_sum(list, int, int) :: any
  def two_sum(_, _, i \\ 0)

  def two_sum([], _, _), do: nil

  def two_sum([cur | rest], target, current_index) do
    case Enum.find_index(rest, fn x -> cur + x == target end) do
      nil -> two_sum(rest, target, current_index + 1)
      i -> [current_index, 1 + current_index + i]
    end
  end
end