public String multiply(String num1, String num2) {
        int length2 = num2.length();
        int length1 = num1.length();
        String[] list = new String[length2+1];
        int total = length1+ length2;
        int i;
        //pre
        for( i=0; i < length2;i++ ){
            list[i] = zero(length2 -i)+ num1+ zero(i);
            list[i] = multip(list[i],num2.charAt(length2-1-i));
        }

        return aftDo(list);
    }