old_contents: class Solution:
      # @param {string} s
      # @param {string} t
      # @return {boolean}
      def isIsomorphic(self, s, t):
  
          if len(s) != len(t):
              return False
  
          charDict = {}
          for i, c in enumerate(s):
              if c not in charDict.keys() and t[i] not in charDict.values():
                  charDict[c] = t[i]
              elif t[i] in charDict.values() or charDict[c] != t[i]:
                  return False
          return True
  
  if __name__ == '__main__':
      test_list = [["ab","aa"],["aa", "bb"]
      result_list = [False, True]
      success = True
      solution = Solution()
      for i, s in enumerate(test_list):
          result = solution.isIsomorphic(s[0], s[1])
          if result != result_list[i]:
              success = False
              print s
              print 'Expected value', result_list[i]
              print 'Actual value', result
      if success:
          print 'All the tests passed.'
      else:
          print 'Please fix the failed test'
  
new_contents: class Solution:
      # @param {string} s
      # @param {string} t
      # @return {boolean}
      def isIsomorphic(self, s, t):
  
          if len(s) != len(t):
              return False
  
          charDict = {}
          for i, c in enumerate(s):
              if c not in charDict.keys() and t[i] not in charDict.values():
                  charDict[c] = t[i]
              elif c in charDict.keys() and charDict[c] != t[i]:
                  return False
              elif t[i] in charDict.values():
                  if c not in charDict.keys():
                      return False
                  elif charDict[c] != t[i]:
                      return False
          return True
  
  if __name__ == '__main__':
      test_list = [["ab","aa"],["aa", "bb"], ["egg", "add"],["foo","bar"],["paper","title"]]
      result_list = [False, True, True, False, True]
      success = True
      solution = Solution()
      for i, s in enumerate(test_list):
          result = solution.isIsomorphic(s[0], s[1])
          if result != result_list[i]:
              success = False
              print s
              print 'Expected value', result_list[i]
              print 'Actual value', result
      if success:
          print 'All the tests passed.'
      else:
          print 'Please fix the failed test'
  
subject: Add solution for the isomorphic strings