For this problem, we are OK to use hash set, since no numbers are needed.

 

1 class Solution { 2 public: 3     int lengthOfLongestSubstring(string s) { 4         int len = s.size(), result = 0; 5         if (len < 2) return len; 6         unordered_set
     
       sets; 7         for (int left = 0, right = 0; right < len; right++) { 8             if (sets.find(s[right]) == sets.end()) { 9                 result = max(result, right - left + 1);10                 sets.insert(s[right]);11             } else {12                 sets.erase(s[left++]);13                 right--;14             }15         }16         return result;17     }18 };