双指针

哈希表记录字符出现次数

Python用字典，设置默认值为0

import collections
dic = collections.defaultdict(lambda :0)   

import collections
# dic = collections.defaultdict(lambda :0)
class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        hash = collections.defaultdict(lambda :0) # 设置默认值为0
        res = 0
        i = 0; j = 0 # j为右指针，i为左指针
        while j < len(s):
            hash[s[j]] += 1
            while hash[s[j]] > 1: 
                # 当右指针指向的字符出现两次时，左指针向右移动直到把前面的重复字符删掉为止
                hash[s[i]] -= 1
                i += 1 
            res = max(res, j - i + 1) 
            j += 1
            # print(hash)
        return res