non consecutive next letter

consecutive.rb

def replace(r)
  starray = r.split("")

  starray.each_with_index.reduce([]) do |word, current|
    letter, index = current
    next_letter = starray[index + 1]

    word <<
      if letter == '?'
        get_next_letter(word.last, next_letter)
      else
        letter
      end
  end.join

end


def get_next_letter(last, next_letter)
  return 'a' unless last
  non_consecutive = last.next
  return non_consecutive if next_letter == '?'

  if non_consecutive != next_letter
    non_consecutive
  else
    get_next_letter(non_consecutive, next_letter)
  end
end


puts replace("????ab?????")
#abcdabcdefg