public String multiply(String num1, String num2) {
        if (num1 == null || num1.length() == 0
                || num2 == null || num2.length() == 0
                || num1.equals("0") || num2.equals("0")) {
            return "0";
        }
        int[] res = mult(num1.toCharArray(), num2.toCharArray());
        StringBuilder buf = new StringBuilder();
        int i;
        for (i = res.length - 1; i >= 0 && res[i] == 0; i --);
        for (; i >= 0; i --) {
            buf.append(res[i]);
        }
        return buf.toString();
    }