input: 
  def num_common_chars(str1: str, str2: str) -> int:
      """
      Given two strings, return 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('aaa', 'a')
      1
      """
      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