Java求数列的和

计算数列：9，99，999，…，9999999999的和

public static void main(String[] args) {
		long sum = 0;
		long num = 0;
		for (int i = 1; i <= 10; i++) { // 10次
			num = num * 10 + 9;
			sum += num;
		}
		System.out.println("sum=" + sum);
	}