P4047 [JSOI2010]部落划分

​​传送门​​

思路：

要使划分后有k坨部落，也就是要进行两两合并n-k次，合并后再找到最近两坨部落间的距离即为答案。

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int inf = 0x3f3f3f3f;
struct node{
  int a,b;
  double t;
  friend bool operator < (node a, node b)
  {
    return a.t > b.t;
  }
};
priority_queue<node>q;
int x[1010],y[1010];
node now;
int f[1010];
int find(int x)
{
  return f[x]==x?x:f[x]=find(f[x]);
}
int main()
{
  int n,k;
  cin>>n>>k;
  for(int i = 1; i <= n; i++)f[i] = i;
  for(int i = 1; i <= n; i++)
  {
    cin>>x[i]>>y[i];
  }
  for(int i = 1; i <= n; i++)
  {
    for(int j = 1; j <= n; j++)
    {
      if(i != j)
      {
        now.a = i;
        now.b = j;
        now.t = sqrt(pow(x[i]-x[j],2)+pow(y[i]-y[j],2));
        q.push(now);
      }
    }
  }
  int cnt = 0;
  double ans = 0x3f3f3f3f;
  while(cnt <= n-k)
  {
    now = q.top();
    q.pop();
    if(find(now.a) != find(now.b))
    {
      cnt++;
//      ans = min(ans,now.t);
      f[find(now.a)] = find(now.b);
    }
  }
  while(q.size())
  {
    now = q.top();
    q.pop();
    if(find(now.a) != now.b)
    {
      ans = now.t;
      break;
    }
  }
  printf("%0.2lf\n",ans);
}