input: Write a python function to find common characters in two strings.
  assert compare_strings("hello", "world") == 'lo'
  assert compare_strings("python", "java") == 'a'
  assert compare_strings("apple", "pineapple") == 'elap'
output: def compare_strings(s1, s2):
      return ''.join(sorted(set(s1) & set(s2)))