defmodule MissingInteger do
  def missing_integer(a) do
    sorted = a |> Enum.uniq() |> Enum.sort()

    Enum.reduce_while(sorted, 1, fn v, min ->
      cond do
        v <= 0 -> {:cont, min}
        min != v -> {:halt, min}
        true -> {:cont, min + 1}
      end
    end)
  end
end