leetcode problem2 (brute force) not all test cases passed

class Solution:
  def lengthOfLongestSubstring(self, s):
    if len(s) == 0:
      return 1
    else:
        self.l = ''
        self.count = 0
        self.mlist = []
        length =0
        for i in s:
            if i not in self.l:
                self.l += i
                self.count +=1
            elif i in self.l:
                self.mlist.append(self.count)
                self.count =0
                self.l = ''
                self.l +=i
                self.count +=1
            #return max(self.mlist)
            for i in self.mlist:
                if i > length:
                    length = i
            return length

a = Solution()
print(a.lengthOfLongestSubstring(""))