array_flattener.rb

  def array_flattener(array)
    raise ArgumentError, 'Argument must an array' unless array.is_a? Array
    flattened_array = []
    array.each do |element|
      if element.is_a? Array
        flattened_array += array_flattener(element)
      else
        flattened_array << element
      end
    end
    flattened_array
  end