input: Write a function to check if two strings are isomorphic.
  assert is_isomorphic("egg", "add") == True
  assert is_isomorphic("foo", "bar") == False
  assert is_isomorphic("paper", "title") == True
output: def is_isomorphic(s1, s2):
      if len(s1) != len(s2):
          return False
  
      mapping_s1_to_s2 = {}
      mapping_s2_to_s1 = {}
  
      for char1, char2 in zip(s1, s2):
          if (char1 in mapping_s1_to_s2 and mapping_s1_to_s2[char1] != char2) or \
             (char2 in mapping_s2_to_s1 and mapping_s2_to_s1[char2] != char1):
              return False
          mapping_s1_to_s2[char1] = char2
          mapping_s2_to_s1[char2] = char1
  
      return True