input: Write a function to determine the total number of common characters present in both strings. If there are no common characters return 0.
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("apple", "orange")==3
  assert num_common_chars("race", "care")==4
  assert num_common_chars("aab", "aabc")==3
  assert num_common_chars("", "test")==0
output: def num_common_chars(s1, s2):
      set1 = set(s1)
      set2 = set(s2)
      common_chars = set1 & set2
      return len(common_chars)