BZOJ 1821 [JSOI2010] Group 部落划分 Group

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
typedef long long LL;
using namespace std;
const int maxn=2100;
int n,k,x[maxn],y[maxn],tot;
double ans;
struct edge{
    int u,v;
    double w;
    friend bool operator <(const edge&a,const edge &b) {
        return a.w<b.w;
     }
}e[1005*1005];
double dis(int a,int b) {
    return sqrt((double)(x[a]-x[b])*(x[a]-x[b])+(double)(y[a]-y[b])*(y[a]-y[b]));
}
int fa[maxn];
int find(int x) {return x==fa[x]?x:fa[x]=find(fa[x]);}
void kruskal() {
    sort(e+1,e+tot+1);
    for(int i=1;i<=n;i++) fa[i]=i;
    int cnt=0;
    for(int i=1;i<=tot;i++) {
        int u=e[i].u,v=e[i].v;
        int fu=find(u),fv=find(v);
        if(fu!=fv) {
            cnt++;
            if(cnt==n-k+1) {
                ans=e[i].w;
                break;
            }
            fa[fu]=fv;
        }
    }
}
int main()
{
    //freopen(".in","r",stdin);
    //freopen(".out","w",stdout);
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++) 
        scanf("%d%d",&x[i],&y[i]);
    for(int i=1;i<=n;i++) 
        for(int j=i+1;j<=n;j++) {
            e[++tot].u=i;
            e[tot].v=j;
            e[tot].w=dis(i,j);
        }
    kruskal();
    printf("%.2lf\n",ans);
    return 0;
}