3

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int ans = 0, start = -1;
        vector<int> last(256, -1);
        for(int i=0; i<s.length(); i++){
            if(last[s[i]] > start)
                start = last[s[i]];
            ans = max(ans, i-start);
            last[s[i]] = i;
        }
        return ans;
    }
};