阶乘与9 99 999 9999 99999...

public class Demo4{
    public static void main(String[] args) {
        int sum = 0;
        /*1!+2!+3!+4!+...+20!
        2!= 2*1!
        3! = 3*2!*/
        int a = 1;
        for (int i=1;i<=20;i++) {
            a *= i;//a = 1  a= 1!*2 = 2!  a= 3*a = 3*2!=3!
            sum += a;
        }
        System.out.println(sum);

        //9 99 999 9999 99999...
        //9 9*10+9  99*10+9
        //9 9*10+9 9*100+99  9*1000+999
        long sum1 = 0;
        long b = 0;
        for (int i=1;i<=10;i++) {
            b = b*10+9;//b= 9 b= 9*10+9=99  b= 99*10+9
            sum1 += b;
        }
        System.out.println(sum1);

    }
}