好像是個稠密圖 總之跑跑prim就過了 注意要求的是第K+1長邊

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<ctime>
#include<cstring>
#include<queue>
using namespace std;
queue<int>Q;
int S, P;
double
 
 dist[505];
double x[505];
double y[505];
int lstot;
double dis[505][505];
double lst[505];
bool vis[505];
inline double calc(const double &deltax, const double &deltay) {
	return sqrt(deltax * deltax + deltay * deltay);
}
int main() {
	int T;
	scanf("%d", &T);
	while (T--) {
		scanf("%d%d", &
 
S, &P);
		for (int i = 1; i <= P; ++i) {
			scanf("%lf%lf", &x[i], &y[i]);
		}
		for (int i = 1; i <= P; ++i) {
			for (int j = 1; j <= P; ++j) {
				dis[i][j] = calc(x[i]-x[j],y[i]-y[j]);
			}
		}
		memset(vis, 0, sizeof(vis));
		Q.push(1);
		lstot = 0;
		vis[1] = 1;
		for (int i =
 
 1; i <= P; ++i) {
			dist[i] = dis[1][i];
		}
		for (int i = 1; i < P; ++i) {
			double mn = 1145143810.0;
			int mnInd = 0;
			for (int j = 1; j <= P; ++j) {
				if (vis[j]) continue;
				if (dist[j] < mn) {
					mn = dist[j];
					mnInd = j;
				}
			}
			vis[mnInd] = 1;
			for (int j = 1; j <= P; ++j) {
				if (vis[j]) continue;
				dist[j] = min(dist[j], dis[mnInd][j]);
			}
			lst[++lstot] = mn;
		}
		sort(lst+1, lst+1+lstot);
		printf("%.2f\n", lst[P-S]);
	}
	return 0;
}