Building a Space Station POJ

#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstdio>
using namespace std;
const int N=1e5;
struct edge{
    int a,b;
    double w;
}e[N];
double x[N],y[N],z[N],r[N];
int n,tot,p[N];
bool cmp(edge a,edge b)
{
    return a.w<b.w;
}
int find(int x)
{
    if(p[x]!=x)
        p[x]=find(p[x]);
    return p[x];
}
void kruskal()
{
    double sum=0;
    for(int i=0;i<tot;i++)
    {
        int a=find(e[i].a);
        int b=find(e[i].b);
        double w=e[i].w;
        if(a!=b)
        {
            sum+=w;
            p[a]=b;
        }
    }
    printf("%.3f
",sum);
}
int main()
{
    while(cin>>n&&n)
    {
        for(int i=0;i<n;i++)
            cin>>x[i]>>y[i]>>z[i]>>r[i];
        for(int i=0;i<=n;i++)
            p[i]=i;
        tot=0;
        for(int i=0;i<n;i++)
            for(int j=i+1;j<n;j++)
            {
                double t=sqrt((x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j])+(z[i]-z[j])*(z[i]-z[j]));
                if(r[i]+r[j]>=t)
                    e[tot++]={i,j,0};
                else
                    e[tot++]={i,j,t-r[i]-r[j]};
            }
            sort(e,e+tot,cmp);
            kruskal();

    } 
    return 0;
}