UVA 10886 Standard Deviation

https://vjudge.net/problem/UVA-10886

计算标准差

碰到这种题将式子展开

#include<cmath>
#include<cstdio>
//#include<iostream>
using namespace std;
unsigned long long seed;
long double gen()
{
    static const long double Z = ( long double )1.0 / (1LL<<32);
    seed >>= 16;
    seed &= ( 1ULL << 32 ) - 1;
    seed *= seed;
    return seed * Z;
}
int main()
{
    int T,n;
    double xi,xi2,y,x;
    scanf("%d",&T);
    for(int t=1;t<=T;t++)
    {
        xi=xi2=0;
        scanf("%d%llu",&n,&seed);
        //cin>>n>>seed;
        for(int i=1;i<=n;i++)
        {
            y=gen();
            xi+=y;
            xi2+=y*y;
        } 
        x=xi/n;
        printf("Case #%d: %.5lf
",t,sqrt((xi2-2*x*xi+n*x*x)/n));
    }
}