defmodule S do
  def test do
    a = [[1, 3], [2, 6], [8, 10], [7, 11]]
    [[1,6], [7,11]] = interval(a)

    a = [[5,12],[8,10]]
    [[5,12]] = interval(a)

    a = [[1,6], [6,10]]
    ^a = interval(a)

    :ok
  end

  def interval(xs) do
    [[a,b]|xs] = Enum.sort(xs, fn([a, _], [b, _]) -> a <= b end)
    merge(xs, a, b, [])
  end

  def merge([], a1, b1, ret), do: [[a1,b1]|ret] |> Enum.reverse

  def merge([[a,b]|xs], a1, b1, ret) do
    case a < b1 do
      true -> merge(xs, a1, max(b1, b), ret)
      false -> merge(xs, a, b, [[a1,b1]|ret])
    end
  end
end