class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int n = s.length();
        int ret = 0;
        unordered_map<char, int> count;
        int l = 0;
        for(int r = 0; r < n; r++){
            count[s[r]]++;
            while(count[s[r]] >= 2){
                count[s[l++]]--;
            }
            ret = max(ret, r - l + 1);
        }
        return ret;
    }
};