input: def num_common_chars(str1: str, str2: str) -> int:
      """
      Given two strings, find the total number of common characters present in both strings.
      >>> num_common_chars('hello', 'world')
      3
      >>> num_common_chars('abc', 'def')
      0
      >>> num_common_chars('aa', 'a')
      1
      """
output:     set1 = set(str1)
      set2 = set(str2)
      common_chars = set1 & set2
      return len(common_chars)