class Solution {
    public int lengthOfLongestSubstring(String S) {
        char[] s = S.toCharArray();
        int n = s.length;
        int ans = 0;
        int left = 0;
        boolean[] has = new boolean[128];
        for(int right = 0; right < n; right ++ ){
            char c = s[right];
            while(has[c])
                has[s[left ++ ]] = false;
            has[c] = true;
            ans = Math.max(ans, right - left + 1);
        }

        return ans;
    }
}