input: Write a function to find the total number of common characters present in both strings.
  
  def num_common_chars(str1, str2):
      common = 0
      for c in str1:
          if c in str2:
              common += 1
      return common
  
  assert num_common_chars("hello", "world") == 3
  assert num_common_chars("a", "b") == 0
  assert num_common_chars("abcdef", "fedcba") == 6
  assert num_common_chars("aaa", "aa") == 2
  assert num_common_chars("", "a") == 0
output: def num_common_chars(str1, str2):
      common = 0
      for c in str1:
          if c in str2:
              common += 1
              str2 = str2.replace(c, '', 1)
      return common