poj 2031Building a Space Station(几何判断+Kruskal最小生成树)

 /*
   最小生成树 + 几何判断
   Kruskal      球心之间的距离 - 两个球的半径 < 0 则说明是覆盖的！此时的距离按照0计算
 */
 #include<iostream>
 #include<cstdio>
 #include<cstring>
 #include<cmath>
 #include<algorithm>
 using namespace std;
 int f[];
 struct ball{
    double x, y, z, r;
 };

 struct connect{
    double dist;
    int a, b;
 };

 connect c[];

 ball b[];

 bool cmp(connect a, connect b){
    return a.dist < b.dist;
 }
 int n;

 int getFather(int x){
    return x==f[x] ? x : f[x]=getFather(f[x]);
 } 

 int Union(int a, int b){
     int fa=getFather(a), fb=getFather(b);
     if(fa!=fb){
         f[fa]=fb;
         return ;
     }
     return ;
 }

 int main(){
    int i, j;
    while(scanf("%d", &n) && n){
       for(i=; i<=n; ++i)
          scanf("%lf%lf%lf%lf", &b[i].x, &b[i].y, &b[i].z, &b[i].r);
       int cnt=;
       for(i=; i<n; ++i)
          for(j=i+; j<=n; ++j){
              double d = sqrt((b[i].x-b[j].x)*(b[i].x-b[j].x) + (b[i].y-b[j].y)*(b[i].y-b[j].y) + (b[i].z-b[j].z)*(b[i].z-b[j].z))
                         - (b[i].r + b[j].r);
                c[cnt].dist= d< ? : d;
                c[cnt].a=i;
                c[cnt++].b=j;
          }
        sort(c, c+cnt, cmp);
        double minSum=0.0;
        for(i=; i<=n; ++i)
           f[i]=i;
        for(i=; i<cnt; ++i){
           if(Union(c[i].a, c[i].b))
              minSum+=c[i].dist;
        }
        printf("%.3lf\n", minSum);
    }
    return ;
 }