class Solution {public:    int lengthOfLongestSubstring(string s) {        int res = 0, left = -1, n = s.size();        unordered_map
    
      m;        for (int i = 0; i < n; ++i) {            if (m.count(s[i]) && m[s[i]] > left) {                left = m[s[i]];              }            m[s[i]] = i;            res = max(res, i - left);                    }        return res;    }};