input: Write a function to find the total number of common characters present in both strings. If the given strings are 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("abc", "def")==0
  assert num_common_chars("aaaa", "aa")==2
  assert num_common_chars("", "test")==0
  assert num_common_chars("python", "")==0
output: def num_common_chars(str1, str2):
      common = 0
      for c in set(str1):
          common += min(str1.count(c), str2.count(c))
      return common