尝试Leetcode问题并超过接收内存?

我正在尝试解决Leetcode上的一个Python问题,该问题要求您计算并返回包含给定字符串的唯一字符的最长子字符串的长度。这是我测试过的代码,似乎可以工作,但Leetcode站点不接受它,并通知我“内存超出”错误。知道怎么解决这个问题吗?

class Solution(object):
    def lengthOfLongestSubstring(self, s):
        """ First access all of the substrings """
        ls_subs = []
        unique_subs = []
        for i in range(len(s)):
            sub = s[i:]
            ls_subs.append(sub)

        """ Build versions of the substrings which contain unique chars """
        for sub in ls_subs:
            counter = 0
            unique = ""
            while counter < len(sub) and sub[counter] not in unique:
                unique += sub[counter]
                counter += 1
            unique_subs.append(unique)


        """ Find the longest one """
        length = 0
        for sub in unique_subs:
            if len(sub) > length:
                length = len(sub)
        return length