class Solution {
    public int lengthOfLongestSubstring(String S) {
        char[] s = S.toCharArray();
        int[] cnt = new int[128];
        int left = 0;
        int res = 0;

        for(int right = 0; right < s.length; right ++ ){
            char c = s[right];
            cnt[c] ++ ;

            while(cnt[c] > 1){
                cnt[s[left]] -- ;
                left ++ ;
            }

            res = Math.max(res, right - left + 1);
        }

        return res;
    }
}