POJ2349(求生成树中符合题意的边)

#include<cstdio>
#include<cmath>
#include<cstring>
#include<queue>
#include<algorithm>
#include<vector>
using namespace std;
const int MAXN=505*505;
const double INF=1.0e8;
typedef pair<double,int> P;
struct Point{
    int x,y,index;
}points[505];
struct Edge{
    int to,next;
    double cost;
}es[MAXN];
int V,E;
double dist(int x1,int y1,int x2,int y2)
{
    return sqrt((double)((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)));
}
bool comp(double a,double b)
{
    return a > b;
}
int head[505];
void add_edge(int u,int v,double cost)
{
    es[E].to=v;
    es[E].cost=cost;
    es[E].next=head[u];
    head[u]=E;
    E++;
}
int cnt;
double res[505];
double d[505];
int vis[505];
void prim(int s)
{
    for(int i=1;i<=V;i++)
    {
        d[i]=INF;
        vis[i]=0;
    }
    d[s]=0;
    cnt=0;
    priority_queue<P,vector<P>,greater<P> > que;
    que.push(P(0,s));
    while(!que.empty())
    {
        P now=que.top();que.pop();
        int v=now.second;
        if(vis[v])    continue;
        res[cnt++]=now.first;
        vis[v]=1;
        for(int i=head[v];i!=-1;i=es[i].next)
        {
            Edge e=es[i];
            if(d[e.to]>e.cost)
            {
                d[e.to]=e.cost;
                que.push(P(d[e.to],e.to));
            }    
        }
    }
}

int main()
{
    int T;
    scanf("%d",&T);
    int S,P;
    while(T--)
    {
        scanf("%d%d",&S,&P);
        memset(head,-1,sizeof(head));
        V=P;
        E=0;
        for(int i=0;i<P;i++)
        {
            scanf("%d%d",&points[i].x,&points[i].y);
            points[i].index=i+1;
        }
        for(int i=0;i<P;i++)
            for(int j=i+1;j<P;j++)
            {
                int indi=points[i].index;
                int indj=points[j].index;
                double co=dist(points[i].x,points[i].y,points[j].x,points[j].y);
                add_edge(indi,indj,co);
                add_edge(indj,indi,co);
            }    
        
        prim(1);
        sort(res,res+cnt,comp);    
        printf("%0.2lf\n",res[S-1]);
    }
    
    return 0;
}