input: 
  def num_common_chars(str1: str, str2: str) -> int:
      """ Given two strings, returns the total number of common characters present in both strings.
      >>> num_common_chars("abc", "def")
      0
      >>> num_common_chars("aabbcc", "abcabc")
      6
      """
      common = 0
      for c in str1:
          if c in str2:
              common += 1
      return common
  
output:     common = 0
      for c in str1:
          if c in str2:
              common += 1
              str2 = str2.replace(c, '', 1)
      return common