Length of Longest Substring Without Repetition

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        n=len(s)
        seen={}
        start=0
        res=0
        for end in range(n):
            if s[end] in seen:
                start=max(start,seen[s[end]]+1)    
            seen[s[end]] = end
            res=max(res,end-start+1)
        return res