Leetcode: Longest Substring Without Repeating Characters @Python

class Solution:
    # @return an integer
    def lengthOfLongestSubstring(self, s):
        start=0
        n=len(s)
        substr=''
        maxlen=0
        for end in range(n):
            if s[end] not in substr:
                substr+=s[end]
            else:
                substr=''
                maxlen=max(maxlen,end-start)
                for i in xrange(start,end):
                    if s[end]==s[i]:
                        start=i+1
                        substr=s[start:end+1]
                        break
        maxlen=max(maxlen,n-start)
        return maxlen