class Solution(object):
    def findTheDifference(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: str
        """
        dic = {}
        for ch in s:
            dic[ch] = dic.get(ch, 0) + 1
        for ch in t:
            if dic.get(ch, 0) == 0:
                return ch
            else:
                dic[ch] -= 1

#Runtime: 28 ms
#Your runtime beats 54.76 % of python submissions.