public String multiply(String num1, String num2) {
        if(num1.equals("0") || num2.equals("0")) return "0";
        String result = new String();
        for(int i = num2.length() - 1; i >= 0; i--){
            Character single = num2.charAt(i);
            StringBuilder product = new StringBuilder(mulitplySingle(num1, single));
            for(int j = 0; j < num2.length() - i - 1; j++)
                product.append("0");
            result = add(result, product.toString());
        }
        return result;
    }