POJ 2031 Building a Space Station

POJ 2031 Building a Space Station

MST问题。
以下是我的代码：

#include < cstdio >
#include < cmath >
using   namespace  std;
const   int  kMaxn( 107 );

int  n;
double  x[kMaxn],y[kMaxn],z[kMaxn],r[kMaxn];
double  g[kMaxn][kMaxn];
double  mst,lowcost[kMaxn];
bool  visited[kMaxn];

double  distance( int  a, int  b)
{
     return  sqrt((x[a] - x[b]) * (x[a] - x[b]) + (y[a] - y[b]) * (y[a] - y[b]) + (z[a] - z[b]) * (z[a] - z[b]));
}

void  Prim()
{
    mst = . 0 ;
     for ( int  i = 1 ;i <= n;i ++ )
    {
        lowcost[i] = g[ 1 ][i];
        visited[i] = false ;
    }
    visited[ 1 ] = true ;

     for ( int  i = 1 ;i < n;i ++ )
    {
         int  v( - 1 );
         double  w( 0x7f7f7f7f + . 0 );
         for ( int  j = 1 ;j <= n;j ++ )
             if ( ! visited[j]  &&  w > lowcost[j])
            {
                v = j;
                w = lowcost[j];
            }
         if (v !=- 1 )
        {
            mst += w;
            visited[v] = true ;
             for ( int  j = 1 ;j <= n;j ++ )
                 if ( ! visited[j]  &&  lowcost[j] > g[v][j])
                    lowcost[j] = g[v][j];
        }
    }
}

int  main()
{
     /*
    freopen("data.in","r",stdin);
    freopen("data.out","w",stdout);
    // */

     while (scanf( " %d " , & n) == 1   &&  n)
    {
         for ( int  i = 1 ;i <= n;i ++ )
        {
            scanf( " %lf%lf%lf%lf " , & x[i], & y[i], & z[i], & r[i]);
             for ( int  j = 1 ;j < i;j ++ )
            {
                 double  t(distance(i,j));
                 if (t > r[i] + r[j])
                    g[i][j] = g[j][i] = t - r[i] - r[j];
                 else
                    g[i][j] = g[j][i] = . 0 ;
            }
        }

        Prim();

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

     return   0 ;
}