POJ 2560

 #include<iostream>
 #include<algorithm>
 #include<cmath>
 #include<iomanip>
 #define MAXN 105
 #define inf 1000000000
 using namespace std;

 typedef double elem_t;

 double point[MAXN][];
 double _m[MAXN][MAXN];
 int pre[MAXN];
 elem_t prim(int n,elem_t mat[][MAXN],int* pre);
 int main()
 {
     //freopen("acm.acm","r",stdin);
     int num;
     int i;
     int j;
     double x;
     double y;
     cin>>num;
     for(i = ; i < num; ++ i)
     {
         cin>>point[i][]>>point[i][];
     }
     for(i = ; i < num; ++ i)
     {
         for(j = i+; j < num; ++ j)
         {
             _m[i][j] = sqrt((point[i][] - point[j][])*(point[i][] - point[j][]) + (point[i][] - point[j][])*(point[i][] - point[j][]));
             _m[j][i] = _m[i][j];
         }
     }
     cout<<setiosflags(ios::fixed)<<setprecision()<<prim(num,_m,pre)<<endl;
 }

 elem_t prim(int n,elem_t mat[][MAXN],int* pre){
     elem_t min[MAXN],ret=;
     int v[MAXN],i,j,k;
     for (i=;i<n;i++)
         min[i]=inf,v[i]=,pre[i]=-;
     for (min[j=]=;j<n;j++){
         for (k=-,i=;i<n;i++)
             if (!v[i]&&(k==-||min[i]<min[k]))
                 k=i;
         for (v[k]=,ret+=min[k],i=;i<n;i++)
             if (!v[i]&&mat[k][i]<min[i])
                 min[i]=mat[pre[i]=k][i];
     }
     return ret;
 }