input: Write a function to find the total number of common characters present in both strings. If both strings are empty, return 0.
  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")==4
  assert num_common_chars("abc", "def")==0
  assert num_common_chars("same", "same")==4
  assert num_common_chars("", "")==0
  assert num_common_chars("common", "unique")==1
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