先给出一个错误答案，这个答案错在解决不了abcb型的测试用例

class Solution:
def lengthOfLongestSubstring(self, s: str) -> int:
if not s:return 0
left = 0#左指针
lookup = set()
n = len(s)
maxlen = 0
curlen = 0#滑动窗口
for i in range(n):
curlen += 1
while s[i] in lookup:
lookup.remove(s[left])
left += 1
curlen -= 1
if curlen > maxlen:maxlen = curlen
lookup.add(s[i])
return maxlen

正确答案，用的滑动窗口的概念，就是一个列表

class Solution:
def lengthOfLongestSubstring(self, s: str) -> int:
if not s:return 0
left = 0#左指针
lookup = set()
n = len(s)
maxlen = 0
curlen = 0#滑动窗口
for i in range(n):
curlen += 1
while s[i] in lookup:
lookup.remove(s[left])
left += 1
curlen -= 1
if curlen > maxlen:maxlen = curlen
lookup.add(s[i])
return maxlen

笔记：
1、不要忘记考虑空字符串
2、基于滑动窗口的特性，最后(c,b,b)是3，虽然有重复，但是也不会超过最大长度子串