defmodule PermCheck do
  def perm_check(a) do
    sorted = Enum.sort(a)
    size = length(sorted)

    ok =
      0..(size - 2)//1
      |> Enum.all?(fn k -> Enum.at(sorted, k) == k + 1 end)

    if ok, do: 1, else: 0
  end
end