Flatten an array without using the Ruby library

flatten.rb

def flatten(in_array, out_array = [])
  in_array.each do |a|
    if a.class == Array
      flatten(a, out_array)
    else
      out_array << a
    end
  end
  
  out_array
end

p flatten([[1, 3], [5, [7, 9]]]) # => [1, 3, 5, 7, 9]