LeetCode-Longest Substring Without Repeating Characters

Given a string s, find the length of the longest substring without repeating characters.

Example 1:

Input: s = "abcabcbb" 
Output: 3 
Explanation: The answer is "abc", with the length of 3. 

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:

        dic, res, start, = {}, 0, 0
        for i, ch in enumerate(s):
            # when char already in dictionary
            if ch in dic:
                # check length from start of string to index
                res = max(res, i-start)
                # update start of string index to the next index
                start = max(start, dic[ch]+1)
            # add/update char to/of dictionary 
            dic[ch] = i
        # answer is either in the begining/middle OR some mid to the end of string
        return max(res, len(s)-start)