input: Write a python function to find common characters between two strings.
  assert common_characters("apple", "grape") == ['a', 'e', 'p']
  assert common_characters("house", "mouse") == ['o', 'u', 'e', 's']
  assert common_characters("abcd", "efgh") == []
output: def common_characters(s1, s2):
      return sorted(list(set(s1) & set(s2)))