public static String convertToTitle(int n) {
        if (0 >= n) return null;
        StringBuffer sb = new StringBuffer();
        int t = n;
        while (t > 0) {
            int reminder = t % 26;
            if (0 == reminder) reminder = 26;
            sb.insert(0, (char)('A' + reminder - 1));
            if (26 == reminder) t = t / 26 - 1;
            else t = t / 26;
        }
        return sb.toString();
    }