leetcode#3. 无重复字符的最长子串

class Solution:   
    def lengthOfLongestSubstring(self, s):
        dic, res, start, = {}, 0, 0
        for i, ch in enumerate(s):
            if ch not in dic or (ch in dic and dic[ch]<start):
                res = max(res, i-start+1)
            else:
                start = dic[ch]+1
            dic[ch] = i
        return res