Elixir Perm Missing Element
defmodule PermMissingElement do
  def perm_missing_element(a) do
    sorted = Enum.sort(a)

    sorted
    |> Enum.with_index()
    |> Enum.find_value(fn {v, k} -> if v != k + 1, do: k + 1 end)
    |> Kernel.||(length(sorted) + 1)
  end
end

This uses the expected sum of 1..N+1 and subtracts the actual sum to find the missing value.