BZOJ 1626: [Usaco2007 Dec]Building Roads

#include<iostream>
#include<cstdio>
#include<cmath>
#define INF 2100000000.0
using namespace std;
int n,m,x[2000],y[2000];
double ans,dis[2000],f[2000][2000];
bool vis[2000];
void prim()
{
  for (int i=1;i<=n;i++)
  {
      dis[i]=INF;
      vis[i]=false;
  }
  dis[1]=0.0;
  for (;;)
  {
      int ch=0; double minx=INF;
      for (int i=1;i<=n;i++)
      if ((!vis[i])&&(dis[i]<minx))
      {
         minx=dis[i];
         ch=i;
    }
    if (ch==0) break;
    vis[ch]=true;
    ans+=dis[ch];
    for (int i=1;i<=n;i++) dis[i]=min(dis[i],f[ch][i]);
  }
}
int main()
{
  scanf("%d%d",&n,&m);
  for (int i=1;i<=n;i++) scanf("%d%d",&x[i],&y[i]);
  for (int i=1;i<=n;i++)
   for (int j=i+1;j<=n;j++)
   {
        double sum=(double)sqrt((double)(x[i]-x[j])*(double)(x[i]-x[j])+(double)(y[i]-y[j])*(double)(y[i]-y[j]));
        f[i][j]=f[j][i]=sum;
   }
  for (int i=1;i<=m;i++)
  {
      int a,b;
      scanf("%d%d",&a,&b);
      f[a][b]=f[b][a]=0.0;
  }
  prim();
  printf("%.2lf\n",ans);
  return 0;
}