for (int i=ns.length-1;i>=0;i--){
System.out.println("i = " + i + ", ns[i] = " + ns[i]);
}

//使用for each 求和
int sum1 = 0;
for (int n : ns) {
sum1 += n;
}
System.out.println(sum1);

double pi_quarter = 0.0;
double flag = 1.0;
for (int i = 1; i < 10000; i += 2, flag = -flag){
pi_quarter += flag / i;
}
System.out.println(pi_quarter * 4);

求派的原理不太懂额