module ArrayUtils
  def self.flatten(array)
    array.inject([]) { |accu, elem|
      if elem.kind_of?(Array)
        accu.concat(flatten(elem))
      else
        accu << elem
      end
    }
  end
end

if __FILE__ == $0
  require 'minitest/autorun'

  class ArrayUtilsTest < Minitest::Test
    def test_it_works
      assert_equal [1, 2, 3, 4], ArrayUtils.flatten([[1,2,[3]],4])
    end
  end
end