class Solution {
    public int lengthOfLongestSubstring(String s) {
        HashSet<Character> set = new HashSet<>();
        int res = 0;
        
        for(int l = 0, r = 0; r < s.length(); r++){
            char c = s.charAt(r);

            while(set.contains(c)){
                char d = s.charAt(l);
                set.remove(d);
                l++;
            }
            set.add(c);
            res = Math.max(res, r - l + 1);
        }

        return res;
    }
}