class Solution(object):
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        max_length = 0
        seen_dict = {}
        sub_start = 0
        for i, c in enumerate(s):
            if c in seen_dict:
                sub_start = seen_dict[c]+1
                for k, v in seen_dict.items():
                    if v < sub_start:
                        del seen_dict[k]
            seen_dict[c] = i
            if len(seen_dict) > max_length:
                max_length = len(seen_dict)
        return max_length