new_contents: class Solution:
      # @param {string} s
      # @param {string} t
      # @return {boolean}
  
      def isIsomorphic(self, s, t):
          d1 = {}
          d2 = {}
          l = len(s)
          for i in xrange(l):
              if d1.setdefault(s[i], t[i]) != t[i]:
                  return False
              if d2.setdefault(t[i], s[i]) != s[i]:
                  return False
          return True
  
subject: add solution for Isomorphic Strings