（python）无重复字符的最长子串--算法

def func(s):
        if s == "":
            return 0
        i = 0
        dict1 = {}
        list1 = []
        for j in range(len(s)):
            if s[j] not in dict1:
                dict1[s[j]] = j
                if j == len(s)-1:
                    list1.append(j-i+1)
            else:
                list1.append(j-i)
                m = dict1[s[j]]
                for key in s[i:dict1[s[j]]+1]:
                    dict1.pop(key)
                dict1[s[j]] = j
                i = m + 1
        return max(list1)

将就着看看吧，差点把自己绕进去。。有什么好的写法请推荐