题解 | 小红的圆移动

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const double PI = acos(-1);
int main()
{
    int n,k;
    cin>>n>>k;
    double ans = 0;
    vector<double>vec;
    for(int i=1;i<=n;i++)
    {
        ll x,y,r;
        cin>>x>>y>>r;
        double dis = x*x+y*y;
        double dis2 = r*r;
        if(dis2>dis)
        {
            vec.push_back((sqrtl(dis2)-sqrtl(dis))*PI*r*r);
        }
    }
    sort(vec.begin(),vec.end());
    for(int i=0;i<vec.size()-k;i++)ans+=vec[i];
    printf("%.10lf\n",ans);

    return 0;
}

简单排序贪心即可