recursive version of beautiful string finder

beautiful_recursive.rb

#s = "ABCDEFABDC".split("")
s = "nlhthgrfdnnlprjtecpdrthigjoqdejsfkasoctjijaoebqlrgaiakfsbljmpibkidjsrtkgrdnqsknbarpabgokbsrfhmeklrle".split("")
ans = 'tsocrpkijgdqnbafhmle'
#s = "CABDBACBAB".split("")
#ans = "DCBA"

h = {}.tap do |h| 
  s.each_with_index do |c,i|
    h[c] ||= []
    h[c] << i
  end 
end

def find_mbs(h, left_bound=-1)
  return "" unless h.any?

  right_bound = h.map do |char, positions|
    positions.select{|p| p >= left_bound}.max
  end.min

  candidates = left_bound.upto(right_bound).map(&:to_i)

  next_char = h.select do |char, positions|
    (positions & candidates).any?
  end.map(&:first).flatten.max

  left_bound = h[next_char].select{|i|i> left_bound && i<=right_bound}.min

  h.delete(next_char)

  return "#{next_char}#{find_mbs(h,left_bound)}"
end

most_beautiful_string = find_mbs(h)
puts most_beautiful_string
puts most_beautiful_string == ans