题解 | #数列求和#

public class Main {
    public static void main(String[] args) {
        long sum = 0;
        long num = 0;

        for(int i = 0 ; i < 10 ; i++)
        {
            num += 9 * Math.pow(10,i);
            sum += num;
        }
        System.out.println(sum);


    }
}