Subseq.rb

def subseq(list)
  list.drop(1).each_with_object([[list.first]]) do |number, sequences|
    (number == sequences.last[-1] + 1) ? sequences.last << number : sequences << [number]
  end.max_by(&:count).tap { |sequence| sequence.clear if sequence.count == 1 }
end

puts 'error 1' if subseq([1,0,1,2,3,0,4,5]) != [0,1,2,3]
puts 'error 2' if subseq([5,6,1,3,2,7]) != [5,6]
puts 'error 3' if subseq([2,3,3,4,5]) != [3,4,5]
puts 'error 4' if subseq([7,6,5,4]) != []