def myproduct(*arrays)
  return [] if arrays.empty?
  return arrays[0].collect{|elem| [elem]} if arrays.size == 1

  head = arrays[0]
  tail = arrays.drop(1)

  head.inject([]) do |x1,y1|
    myproduct(*tail).inject(x1) do |x2,y2|
      x2 << [y1] + y2
    end
  end
end

p myproduct([1,2],[3,4])    # => [[1, 3], [1, 4], [2, 3], [2, 4]]
p myproduct([1],[2,3,4])    # => [[1, 2], [1, 3], [1, 4]]
p myproduct([1,2],[3,4],[]) # => []