input: Write a function to find the total number of common characters present in both strings. If either string is empty then 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")==2
  assert num_common_chars("python", "java")==1
  assert num_common_chars("abc", "def")==0
  assert num_common_chars("", "hello")==0
  assert num_common_chars("hello", "")==0
  
output: def num_common_chars(str1, str2):
      if not str1 or not str2:
          return 0
      common = 0
      for c in set(str1):
          if c in str2:
              common += min(str1.count(c), str2.count(c))
      return common