leetcode-3-longest substring without repeating characters

class Solution(object):
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        if not s:
            return 0
        pos = {}
        max_len = 0
        start = 0
        # start ~ index-1 一定是不重复的
        for index, e in enumerate(s):
            if e not in pos or pos[e] < start:
                pos[e] = index
            else:
                max_len = max(index-start, max_len)  # 当前不重复的长度: index-1-(start-1)
                start = pos[e] + 1
                pos[e] = index
        max_len = max(index-start+1, max_len)  # 当前不重复的长度 index - (start-1)
        return max_len