class Solution {
    public int maxFreq(String S, int maxLetters, int minSize, int maxSize) {
        char[] s = S.toCharArray();
        var cnt = new HashMap<Character, Integer>();
        var res = new HashMap<String, Integer>();
        for(int i = 0; i < minSize - 1; i ++ ) cnt.merge(s[i], 1, Integer::sum);

        for(int i = minSize - 1; i < s.length; i ++ ){
            cnt.merge(s[i], 1, Integer::sum);

            if(cnt.size() <= maxLetters) {
                res.merge(S.substring(i - minSize + 1 ,i + 1), 1, Integer::sum);
            }

            char out = s[i - minSize + 1];
            if(cnt.merge(out, -1, Integer::sum) == 0)
                cnt.remove(out);
        }

        int ans = 0;
        for(Integer value: res.values()){
            ans = Math.max(ans, value);
        }

        return ans;
    }
}