input: Write a python function to count the number of common characters between two strings.
  assert num_common_chars("hello", "world") == 3
  assert num_common_chars("abc", "def") == 0
  assert num_common_chars("python", "typhon") == 5
output: def num_common_chars(s1, s2):
      return len(set(s1) & set(s2))