第三课作业3:1!+1/2!+1/3!+1/4!+....的前20项和

For语句：

public class hw3{
public static void main(String args[]){
double x,y=1,z=0;
for(x=1;x<=20;x++){
	y=1/x*y;
	z=z+y;
	System.out.println(z);
}
}
}

While语句：

public class hw3{
public static void main(String args[]){
double x=1,y=1,z=0;
do{
	y=1/x*y;
	z=z+y;
	x++;
}
while(x<=20);
	System.out.println(z);
}
}