双指针

class Solution {
public:
    int longestSubstringWithoutDuplication(string s) {
        unordered_map<char, int> hash;

        int res = 0;
        for (int i = 0, j = 0; i < s.size(); i ++ )
        {
            hash[s[i]] ++ ;             //没有重复i就一直增加，j不变

            while (hash[s[i]] > 1)      //发现重复，j开始追赶i，直到没有重复
                hash[s[j ++ ]] -- ;

            res = max(res, i - j + 1);
        }

        return res;
    }
};