AcWing1145 北极通讯网络

#include<bits/stdc++.h>
#define x first
#define y second
using namespace std;
typedef pair<int,int> pll;
const int N=510000;
struct node{
    int a,b;
    double c;
    bool operator <(const node &t) const{
        return c<t.c;
    }
}e[N];
pll q[N];
int p[N];
double get(pll a,pll b){
    int x=a.x-b.x;
    int y=a.y-b.y;
    return sqrt(x*x+y*y);
}
int find(int x){
    if(x!=p[x]){
        p[x]=find(p[x]);
    }
    return p[x];
}
int main(){
    int n,k;
    int i,j;
    cin>>n>>k;
    for(i=1;i<=n;i++){
        cin>>q[i].x>>q[i].y;
    }
    for(i=1;i<=n;i++)
    p[i]=i;
    int cnt=0;
    for(i=1;i<=n;i++){
        for(j=1;j<i;j++){
            e[cnt++]=node{i,j,get(q[i],q[j])};
        }
    }
    sort(e,e+cnt);
    double res=0;
    int tmp=n;
    for(i=0;i<cnt;i++){
        if(tmp<=k)
        break;
        int pa=find(e[i].a),pb=find(e[i].b);
        if(pa!=pb){
            p[pa]=pb;
            tmp--;
            res=e[i].c;
        }
    }
    printf("%.2f\n",res);
}