require 'awesome_print'

class Array
  def flatten_2
    reduce([]) do |acc, sub_a|
      if sub_a.is_a?(Array)
        acc + sub_a.flatten_2
      else
        acc << sub_a
      end
    end
  end
end

a = [1, 2, [3, 4, [5, 6, [], 7], [8, 9]], 10, 11]
ap a.flatten == a.flatten_2