public String mulitplySingle(String num, char single){
        int index = num.length() - 1;
        int singleNum = Character.getNumericValue(single);
        StringBuilder result = new StringBuilder();
        int extra = 0;
        while(index >= 0){
            int first = Character.getNumericValue(num.charAt(index));
            int digit = (first * singleNum + extra) % 10;
            extra = (first * singleNum + extra) / 10;
            result.append(digit);
            index--;
        }
        if(extra > 0)
            result.append(extra);
        return result.reverse().toString();
    }