;struct d{int u,v;double w;} e[10000+5];bool cmp(d a,d b){return a.w<b.w;}int bin[100+5];double x[100+5],y[100+5],z[100+5],r[100+5];int Find(int x){int s;for(s=x; bin[s]>=0; s=bin[s]);while(s!=x){int t=bin[x];bin[x]=s;x=t;}return s;}void Union(int x1,int x2){int f1=Find(x1),f2=Find(x2);int t=bin[f1]+bin[f2];if(bin[f1]>bin[f2]){bin[f1]=f2;bin[f2]=t;}else{bin[f2]=f1;bin[f1]=t;}}int main(){int n,m,i,j,u,v;double ans;while(~scanf(“%d”,&n)&&n){for(i=0; i<n; i++)bin[i]=-1;for(i=0; i<n; i++)scanf(“%lf%lf%lf%lf”,&x[i],&y[i],&z[i],&r[i]);for(i=0,m=0; i<n; i++)for(j=i+1; j<n; j++){double t=(x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j])+(z[i]-z[j])*(z[i]-z[j]);t=sqrt(t);if(t<=r[i]+r[j]){if(Find(i)!=Find(j)) Union(i,j);}else{e[m].u=i;e[m].v=j;e[m].w=t-r[i]-r[j];m++;}}sort(e,e+m,cmp);for(ans=0,i=0; i<m; i++){u=e[i].u;v=e[i].v;if(Find(u)!=Find(v)){ans+=e[i].w;Union(u,v);}}printf(“%.3f\n”,ans);}return 0;}
，逆境磨练人、逆境是老师、逆境之苦可变甜。
