input: def num_common_chars(str1: str, str2: str) -> int:
      """
      Find the total number of common characters present in both strings.
      >>> num_common_chars("apple", "banana")
      2
      >>> num_common_chars("hello", "world")
      2
      """
output:     set1 = set(str1)
      set2 = set(str2)
      return len(set1 & set2)