poj 2031 Building a Space Station MST

/*
author:jxy
lang:C/C++
university:China,Xidian University
**If you need to reprint,please indicate the source**
*/
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#define INF 1E9
using namespace std;
struct node
{
double x,y,z,r;
};
double d(node a,node b)
{
double t=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y)+(a.z-b.z)*(a.z-b.z))-a.r-b.r;
return t>0?t:0;
}
node place[101];
bool vis[101];
double dis[101];
int main()
{
int n;
while(~scanf("%d",&n)&&n)
{
memset(vis,0,sizeof(vis));
memset(dis,127,sizeof(dis));
int i,j;
for(i=0;i<n;i++)
scanf("%lf%lf%lf%lf",&place[i].x,&place[i].y,&place[i].z,&place[i].r);
double ans=0,Min,t;
int now=0;
for(i=0;i<n;i++)
{
Min=INF;
for(j=0;j<n;j++)
{
if(dis[j]>=Min||vis[j])continue;
Min=dis[j];now=j;
}
vis[now]=1;
if(now)ans+=Min;
for(j=0;j<n;j++)
{
if(vis[j])continue;
dis[j]=min(dis[j],d(place[now],place[j]));
}

}
printf("%.3f\n",ans);
}
}