Time Complexity - O(n log n)

Space Complexity - O(n)

- n = length of s

```
def frequency_sort(s)
result = ""
letter_freq = Hash.new { |h, k| h[k] = 0}
s.each_char do |c|
letter_freq[c] += 1
end
letters = letter_freq.keys.sort_by { |k| letter_freq[k] }.reverse
letters.each do |letter|
result += letter * letter_freq[letter]
end
result
end
```