numbers.rb

class Numbers
  def initialize(str) 
    @str = str
    @str_arr = str.split("").map(&:to_i) #создаю массив цифр из строки
  end

  def take_sequence
    @mega_result = Array.new
    @str_arr.each_index do |i|
      result = Array.new
      if @str_arr.length - 1 == i
        return
      else
        if @str_arr[i] == @str_arr[i + 1] - 1
          @mega_result << result.push(@str_arr[i], @str_arr[i + 1])
        else
          next
        end
      end
    end
    @mega_result
  end
end

p Numbers.new("1235689").take_sequence