Sort word by repeating letters

gistfile1.txt

# @param {String} s
# @return {String}
def frequency_sort(s)
  letter_count = {}

  s.split('').each do |letter|
    if !letter_count[letter].nil?
      letter_count[letter] = letter_count[letter] + 1
    else
      letter_count[letter] = 1
    end
  end

  letter_count = letter_count.sort_by {|k, v| -v}
  new_string = ""

  letter_count.each do |letter_set|
    letter_set[1].times do 
      new_string = new_string + letter_set[0]
    end
  end

  return new_string
  puts "letter_count: #{letter_count}"
end