（最小生成树） Arctic Network -- POJ --2349

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <algorithm>
using namespace std;

const int N = 510;
const int INF = 0xfffffff;

struct node
{
    int x, y;
}s[N];


int n;
double ss[N];
double J[N][N], dist[N];
bool vis[N];

void Prim()
{
    int i, j;
    dist[1]=0;
    memset(vis, 0, sizeof(vis));
    vis[1]=1;

    for(i=1; i<=n; i++)
        dist[i]=J[1][i];

    for(i=1; i<n; i++)
    {
        int index=1;
        double MIN=INF;
        for(j=1; j<=n; j++)
        {
            if(!vis[j] && dist[j]<MIN && dist[j])
            {
                index=j;
                MIN=dist[j];
            }
        }
        vis[index]=1;
        ss[index] = MIN;
        for(j=1; j<=n; j++)
        {
            if(!vis[j] && dist[j]>J[index][j])
                dist[j]=J[index][j];
        }
    }
}

int main ()
{
    int t;
    scanf("%d", &t);
    while(t--)
    {
        int m, i, j;

        scanf("%d%d", &m, &n);

        memset(s, 0, sizeof(s));
        memset(ss, 0, sizeof(ss));

        for(i=1; i<=n; i++)
            scanf("%d%d", &s[i].x, &s[i].y);

        for(i=1; i<=n; i++)
        for(j=1; j<=n; j++)
        {
            double k=sqrt(1.0*(s[i].x-s[j].x)*(s[i].x-s[j].x)+1.0*(s[i].y-s[j].y)*(s[i].y-s[j].y));
            J[i][j]=J[j][i]=k;
        }

        Prim();

        sort(ss+1, ss+n+1);

        printf("%.2f\n", ss[n-m+1]);

    }
    return 0;
}