3 - Longest Substring Without Repeating Characters

from collections import defaultdict
class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        char_dict = defaultdict(int)
        start = ret = 0
        for i, c in enumerate(s):
            char_dict[c] += 1
            while char_dict[c] > 1:
                char_dict[s[start]] -= 1
                start += 1
            ret = max(ret, i - start + 1)
        return ret