poj 2349 Arctic Network

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
#define INF 0x3f3f3f3f
#define max(a, b)(a > b ? a : b)
#define min(a, b)(a < b ? a : b)
#define N 510

using namespace std;

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

double G[N][N], dist[N];
int n;
bool vis[N];

void Init()
{
    int i, j;
    memset(vis, false, sizeof(vis));
    for(i = 0 ; i < n ; i++)
    {
        for(j = 0 ; j < n ; j++)
        {
            if(i == j)
                G[i][j] = 0;
            G[i][j] = G[j][i] = INF;
        }
    }
}

void prim(int s)
{
    int index, i, j;
    double Min;
    for(i = 0; i < n ; i++)
        dist[i] = G[s][i];
    vis[s] = true;
    for(i = 1 ; i < n ; i++)
    {
        Min = INF;
        for(j = 0 ; j < n ; j++)
        {
            if(!vis[j] && dist[j] < Min)
            {
                Min = dist[j];
                index = j;
            }
        }
        vis[index] = true;
        for(j = 0 ; j < n ; j++)
        {
            if(!vis[j] && dist[j] > G[index][j])
                dist[j] = G[index][j];
        }
    }
}

int cmp(const void *a, const void *b)
{
    return *(double *)a > *(double *)b ? 1 : -1;
}//double类型快排，因为这wa了很多次

int main()
{
    int i, j, s, t;
    scanf("%d", &t);
    while(t--)
    {
        scanf("%d%d", &s, &n);
        Init();
        for(i = 0 ; i < n ; i++)
            scanf("%d%d", &node[i].x, &node[i].y);
        for(i = 0 ; i < n ; i++)
        {
            for(j = 0 ; j < n ; j++)
            {
                G[i][j] = G[j][i] = sqrt((node[i].x - node[j].x) * (node[i].x - node[j].x) + (node[i].y - node[j].y) * (node[i].y - node[j].y));
            }
        }
        prim(0);
        qsort(dist, n, sizeof(dist[0]), cmp);
        printf("%.2f\n", dist[n - s]);
    }
    return 0;
}