3. Longest Substring Without Repeating Characters

class Solution(object):
    def lengthOfLongestSubstring(self, s):
        dict={}
        res=0
        start=0
        for i,ch in enumerate(s):
            if ch in dict:
                res=max(res,i-start)
                start=max(start, dict[ch]+1)
            dict[ch]=i
        return max(res,len(s)-start)

       注意enumerate 的用法