input: Write a function to find the total number of common characters present in both strings. If the 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("test", "rest")==3
  assert num_common_chars("", "python")==0
  assert num_common_chars("algorithm", "python")==2
  assert num_common_chars("", "")==0
output: def num_common_chars(str1, str2):
      common = 0
      seen = set()
      for c in str1:
          if c in str2 and c not in seen:
              common += 1
              seen.add(c)
      return common