双指针算法
class Solution {
public:
    int longestSubstringWithoutDuplication(string s) {
        if(!s.size()) return 0;
        unordered_map<char, int> h;
        int res = 0;

        for(int i = 0, j = 0; j < s.size(); j ++){
            h[s[j]] ++;
            if(h[s[j]] > 1){
                while(h[s[i]] == 1) h[s[i ++]] --; 
                i ++;
                h[s[j]] = 1;
            }
            res = max(res, j - i + 1);
        }
        return res;
    }
};