Prim 差错

#include<bits/stdc++.h>
using namespace std;

int n;
double a[5118][5118],ds[5118];
double res;
int x[5118],y[5118];
bool vis[5118];

double dis(int i,int j)
{	return sqrt(double((x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j])));}
int main()
{
//	freopen("c.in","r",stdin);
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
	{
		scanf("%d%d",&x[i],&y[i]);
		for(int j=1;j<i;j++)
			a[i][j]=a[j][i]=dis(i,j);
		ds[i]=123456789.000;
	}
	//prim吧......
	ds[0]=123456789.000;ds[1]=0.0000;
	for(int i=1;i<=n;i++)
	{
		int best=0;
		for(int j=1;j<=n;j++)
			if(ds[j]<ds[best]&&!vis[j]) best=j;
		vis[best]=1;	res+=ds[best];
		for(int j=1;j<=n;j++)
			if(ds[j]>a[best][j]&&!vis[j])
				ds[j]=a[best][j];
	}
	printf("%.2f\n",res);
	return 0;
}