class Array
  def custom_flatten
  flattened = []
    each do |elem|
      flattened += elem.is_a?(Array) ? elem.custom_flatten : [elem]
    end
    flattened
  end
end

RSpec Array
  describe "#custom_flatten"
    let(:ar) { [1,2, [3,4, 5], 5] }

    it "flattens the array into a single one" do
      expect(ar.custom_flatten).to eq([1,2,3,4,5,5])
    end
  end
end