new_contents: class Solution(object):
      def findTheDifference(self, s, t):
          """
          :type s: str
          :type t: str
          :rtype: str
          """
          diff = {}
          for w in s:
              diff[w] = diff.get(w, 0) + 1
          for w in t:
              if diff.get(w, 0) == 0:
                  return w
              diff[w] -= 1
  
subject: Create FindtheDifference_Hash.py