code: def is_isomorphic(s, t):
      
      if len(s) != len(t):
          return False
      dict = {}
      set_value = set()
      for i in range(len(s)):
          if s[i] not in dict:
              if t[i] in set_value:
                  return False
              dict[s[i]] = t[i]
              set_value.add(t[i])
          else:
              if dict[s[i]] != t[i]:
                  return False
      return True
docs: :type s: str
      :type t: str
      :rtype: bool