3 longest substring without repeating characters

class (object):
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        maxLen = 0
        stack = []
        for char in s:
            if char in stack:
                i = stack.index(char)
                if len(stack)>maxLen:
                    maxLen = len(stack)
                stack = stack[i+1:]
                stack.append(char)
            else:
                stack.append(char)
        if len(stack)>maxLen:
            maxLen = len(stack)
        return maxLen

used = {}
max_length = start = 0
for i, c in enumerate(s):
    if c in used and start <= used[c]:
        start = used[c] + 1
    else:
        max_length = max(max_length, i - start + 1)
    used[c] = i
return max_length