poj Building a Space Station

http://poj.org/problem?id=2031

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;

const double eps=1e-8;
const int inf=1<<23;
int cmp(double x)
{
    if(fabs(x)<eps) return 0;
    if(x>0) return 1;
    return -1;
}

const double pi=acos(-1.0);
inline double sqr(double x)
{
    return x*x;
}

inline double Sqrt(double a)
{
    return a<=0?0:sqrt(a);
}

struct point
{
   double x,y,z,r;
   point(){}
   point(double a,double b,double c,double d):x(a),y(b),z(c),r(d){}
};

double dis(const point &a,const point &b)
{
    return Sqrt(sqr(a.x-b.x)+sqr(a.y-b.y)+sqr(a.z-b.z));
}

double dist[1000];
double diss[110][110],ans;
bool vis[1000];
bool prime(int n)
{
    memset(vis,0,sizeof(vis));
    for(int i=1; i<=n; i++)
        dist[i]=inf;
    ans=0;dist[1]=0;
    for(int i=1; i<=n; i++){
        double temp=inf;
        int k=0;
        for(int j=1; j<=n; j++)
        {
            if(!vis[j]&&dist[j]<temp)
            {
                temp=dist[j];
                k=j;
            }
        }
        if(temp==inf) return false;
        vis[k]=true;
        ans+=temp;
        for(int j=1; j<=n; j++)
        {
            if(!vis[j]&&dist[j]>diss[k][j])
            {
                dist[j]=diss[k][j];
            }
        }
    }
    return true;
}
int main()
{
    int n;
    while(scanf("%d",&n)&&n)
    {
        point a[1000];
        memset(diss,0,sizeof(diss));
        for(int i=1; i<=n; i++)
        {
            scanf("%lf%lf%lf%lf",&a[i].x,&a[i].y,&a[i].z,&a[i].r);
        }
        memset(diss,0,sizeof(diss));
        for(int i=1; i<=n; i++)
        {
            for(int j=1; j<=n; j++)
            {
                if(dis(a[i],a[j])-a[i].r-a[j].r<=0)
                diss[i][j]=0;
                else if(dis(a[i],a[j])-a[i].r-a[j].r>eps)
                diss[i][j]=dis(a[i],a[j])-a[i].r-a[j].r;
            }
        }
        prime(n);
        printf("%.3lf
",ans);
    }
    return 0;
}