题解 | 数列求和

import java.util.ArrayList;

public class Main
{
    public static void main(String[] args)
    {
        ArrayList<Long> list = new ArrayList<>();
        double counter = 0;
        long sum = 0;
        for(long i = 9;i <= 9999999999L;i += 9*(long)Math.pow(10,counter))
        {
            list.add(i);
            counter++;
        }
        for(long res : list)
        {
            sum += res;
        }
        System.out.println(sum);
    }
}