input: Write a python function to find common characters between two strings.
  assert commonCharacters("apple", "plea") == 4
  assert commonCharacters("abc", "def") == 0
  assert commonCharacters("hello", "world") == 2
output: def commonCharacters(s1, s2):
      set1 = set(s1)
      set2 = set(s2)
      common_set = set1 & set2
      return len(common_set)