Isomorphic Strings

class Solution(object):
    def isIsomorphic(self, s, t):
        # 풀이
        D = defaultdict(str)
        D2 = defaultdict(str)

        for a, b in zip(s, t):
            if D[a] != '' and D[a] != b:
                return False
            D[a] = b

        for a, b in zip(t, s):
            if D2[a] != '' and D2[a] != b:
                return False
            D2[a] = b
        return True