luogu 4047 [JSOI2010]部落划分 最小生成树

最小生成树或者二分都行，但是最小生成树会好写一些~

Code:

#include <bits/stdc++.h>
#define ll long long
#define N 1000005
#define setIO(s) freopen(s".in","r",stdin)
using namespace std;
ll x[N],y[N];
int p[N];
struct Edge
{
    int u,v;
    ll c;
}e[N];
bool cmp(Edge a,Edge b)
{
    return a.c<b.c;
}
int find(int x)
{
    return p[x]==x?x:p[x]=find(p[x]);
}
int main()
{
    // setIO("input");
    int n,k,i,j,cnt=0,jj;
    scanf("%d%d",&n,&k);
    for(i=1;i<=n;++i) scanf("%lld%lld",&x[i],&y[i]);
    for(i=1;i<=n;++i) p[i]=i;
    for(i=1;i<=n;++i)
    {
        for(j=i+1;j<=n;++j)
        {
            ++cnt;
            e[cnt].u=i;
            e[cnt].v=j;
            e[cnt].c=(x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j]);
        }
    }
    sort(e+1,e+1+cnt,cmp);
    ll lst=0;
    int pp=n;
    lst=e[1].c;
    for(i=1;i<=cnt;i=j)
    {
        for(j=i;j<=cnt&&e[j].c==e[i].c;++j);
        for(jj=i;jj<j;++jj)
        {
            int u=e[jj].u;
            int v=e[jj].v;
            u=find(u), v=find(v);
            if(u!=v) p[u]=v, --pp;
            // printf("%d %d %lld\n",e[jj].u,e[jj].v,e[jj].c);
        }
        if(pp>=k)
        {
            lst=max(lst,e[j].c);
        }
        else
        {
            break;
        }
    }
    // printf("%lld\n",lst);
    printf("%.2f\n",sqrt(lst));
    return 0;
}