leetcode之前提交的代码不见了，所以在这里记一下
https://leetcode.com/problems/longest-substring-without-repeating-characters/
class Solution(object):
def lengthOfLongestSubstring(self, s):
"""
:type s: str
:rtype: int
"""
rtv = start = current = 0
last_seen = {}
while current < len(s):
char = s[current]
if char in last_seen:
start = max(start, last_seen[char] + 1)
if current - start + 1 > rtv:
rtv = current - start + 1
last_seen[char] = current
current += 1
return rtv