class Solution:
    def findTheDifference(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: str
        """
        # method one 集合運算（考慮不到加入相同字母的情況）
        # return "".join(set(t). - set(s))


        # method two   一個蠢方法
        # dic = {}
        # for i in s:
        #     if i not in dic.keys():
 

        #         dic[i] = 1
        #     else:
        #         dic[i] = dic.get(i) + 1
        # for j in t:
        #     if dic.get(j) != t.count(j):
        #         return j

        # method three 呼叫預設的counter()類 可以直接構建字典。
        # http://www.pythoner.com/205.html  collections模組學習資料
        return
 
 list(collections.Counter(t)-collections.Counter(s))[0]