leetcode 3. longest substring without repeating characters [medium]

class  {
    public int lengthOfLongestSubstring(String s) {
        int[] freq = new int[256];
        int l = 0;
        int r = -1;
        int res = 0;

        while (l < s.length()) {
            if (r + 1 >= s.length()) {
                break;
            } else if (r + 1 < s.length() && freq[s.charAt(r + 1)] == 0) {
                freq[s.charAt(++r)]++;
            } else {
                freq[s.charAt(l++)]--;
            }

            res = max(res, r - l + 1);
        }

        return res;
    }

    private int max(int a, int b) {
        return a > b ? a : b;
    }
}