public ArrayList<String> rec(String s) {
        ArrayList<String> al = new ArrayList<String>();

        File f = new File(s);

        if (f.isDirectory()) {
            String[] ls = f.list();

            for (String aux : ls) {
                al.addAll(rec(s + "/" + aux));
            }

        } else {
            if (f.isFile() && s.endsWith("words_mixed.html")) {
                al.add(s);
            }
        }
        return al;
    }