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class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int l = 0, r = 0, res = 0;
        vector<int> f(256, 0);
        while (l < s.size()) {
            if (r < s.size() && f[s[r]] == 0) {
                f[s[r++]]++;
            }
            else {
                f[s[l++]]--;
            }
            res = max(res, r - l);
        }
        return res;
    }
};