Building a Space Station---poj2031

#include<stdio.h>
#include<string.h>
#include<map>
#include<iostream>
#include<algorithm>
#include<math.h>
#define N 110
#define INF 0xfffffff

using namespace std;

int vis[N], n;
double maps[N][N], dist[N];

struct node
{
    double x, y, z, r;
}a[N*N];

void Init()
{
    int i;
    memset(a, 0,sizeof(a));
    memset(vis, 0 ,sizeof(vis));
    for(i=0; i<=n; i++)
    {
        dist[i] = INF;
        for(int j=0; j<=n; j++)
        {
            maps[i][j] = INF;
        }
        maps[i][i] = 0;
    }
}

double Prim(int start)
{
    double ans = 0;
    vis[start] = 1;
    for(int i=1; i<=n; i++)
        dist[i] = maps[start][i];
    for(int i=1; i<=n; i++)
    {
        double Min = INF;
        int index = -1;
        for(int j=1; j<=n; j++)
        {
            if(vis[j] ==0 && Min > dist[j])
            {
                Min = dist[j];
                index = j;
            }
        }
        if(index == -1)break;
        vis[index] = 1;
        ans += Min;
        for(int j=1; j<=n; j++)
        {
            if(vis[j] == 0 && dist[j] > maps[index][j])
                dist[j] = maps[index][j];
        }
    }
    return ans;
}

int main()
{
    int i, j;
    double ans, x, y, z, d;
    while(scanf("%d", &n), n)
    {
        Init();
        for(i=1; i<=n; i++)
        {
            scanf("%lf%lf%lf%lf", &a[i].x, &a[i].y, &a[i].z, &a[i].r);
        }
        for(i=1; i<=n; i++)
        {
            for(j=i+1; j<=n; j++)
            {
                x = a[i].x - a[j].x;
                y = a[i].y - a[j].y;
                z = a[i].z - a[j].z;
                d = sqrt( x*x + y*y + z*z );
                if(d > a[i].r + a[j].r)
                {
                    maps[i][j] = maps[j][i] = d - a[i].r - a[j].r;
                }
                else
                {
                    maps[i][j] = maps[j][i] = 0;
                }
            }
        }
        ans = Prim(1);
        printf("%.3lf\n", ans);
    }
    return 0;
}