class Solution(object):
    def findTheDifference(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: str
        """
        # O(n) – ops complexity
        # O(S) – (S = amount of chars) memory complexity
        cc = {}
        ct = {}
        for c in s:
            cc[c] = cc.get(c, 0) + 1
        for c in t:
            ct[c] = ct.get(c, 0) + 1
        for k, v in ct.items():
            if cc.get(k, -1) != v:
                return k
        return