MultiplyBigNumber

package Fundamentals.TextProcessing.Exercise;

import java.util.Scanner;

public class MultiplyBigNumber2 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);


        String number = scanner.nextLine();
        int multiplier = Integer.parseInt(scanner.nextLine());
        StringBuilder builder = new StringBuilder();
        int sum = 0;
        int remaining = 0;

        for (int i = number.length() - 1; i >= 0; i--) {
            sum = 0;
            if (remaining != 0) {
                sum += remaining;
                remaining = 0;
            }

            sum += Integer.parseInt(String.valueOf(number.charAt(i))) * multiplier;
            if (sum > 9) {
                remaining = sum / 10;
                sum = sum % 10;
            }
            if (i == 0 && remaining != 0) {
                builder.append(sum);
                builder.append(remaining);
                break;
            }
            builder.append(sum);
        }
        builder.reverse();
        System.out.println(builder);
    }
}