input: Write a function to find the total number of common characters present in both strings. If both 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")==3
  assert num_common_chars("python", "java")==1
  assert num_common_chars("abc", "def")==0
  assert num_common_chars("", "")==0
  assert num_common_chars("abc", "abc")==3
output: def num_common_chars(str1, str2):
      common = 0
      for c in set(str1):
          common += min(str1.count(c), str2.count(c))
      return common