poj 2031Building a Space Station

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

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
const int inf=1<<23;
#define maxn 110
using namespace std;

struct point
{
    double x,y,z;
    double r;
}p[110];
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<=0?0: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=0; 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=0; 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=0; 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=0; i<n; i++)
        {
            scanf("%lf%lf%lf%lf",&p[i].x,&p[i].y,&p[i].z,&p[i].r);
        }
        for(int i=0; i<n; i++)
        {
            for(int j=0; j<n; j++)
            {
                if(dis(p[i],p[j])<=0)
                {
                    g[i][j]=0;
                }
                else g[i][j]=dis(p[i],p[j]);
            }
        }
        prim();
        printf("%.3lf\n",ans);
    }
    return 0;
}