poj 2031Building a Space Station

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

 #include<cstdio>
 #include<cstring>
 #include<cmath>
 #include<algorithm>
 const int inf=<<;
 #define maxn 110
 using namespace std;

 struct point
 {
     double x,y,z;
     double r;
 }p[];
 int n;
 double g[maxn][maxn];
 bool vis[maxn];
 double dist[maxn],ans;

 double sqr(double a)
 {
     return a*a;
 }

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

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

 bool prim()
 {
     memset(vis,false,sizeof(vis));
     for(int i=; i<n; i++)
         dist[i]=inf;
      ans=;dist[]=;
     for(int i=; i<=n; i++)
     {
         double temp=inf;
         int k=;
         for(int j=; 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=; j<n; j++)
         {
             if(!vis[j]&&dist[j]>g[k][j])
                 dist[j]=g[k][j];
         }
     }
     return false;
 }

 int main()
 {
     while(scanf("%d",&n)&&n){
         memset(g,inf,sizeof(g));
         for(int i=; i<n; i++)
         {
             scanf("%lf%lf%lf%lf",&p[i].x,&p[i].y,&p[i].z,&p[i].r);
         }
         for(int i=; i<n; i++)
         {
             for(int j=; j<n; j++)
             {
                 if(dis(p[i],p[j])<=)
                 {
                     g[i][j]=;
                 }
                 else g[i][j]=dis(p[i],p[j]);
             }
         }
         prim();
         printf("%.3lf\n",ans);
     }
     return ;
 }