题解 | 小红的圆移动

#include <bits/stdc++.h>
using namespace std;
struct ing {
    int x;
    int y;
    int r;
    double p;
    int flag = 1;
};
bool comp(const ing& a, const ing& b) {
    return a.p < b.p;
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    double tot = 0;
    int x, y, r;
    int n, k;
    int cnt = 0;
    double tep;
    double teq;
	int cnu = 0;
    ing str[100010];
    cin >> n >> k;
    for (int i = 0; i < n; i++) {
        cin >> str[i].x >> str[i].y >> str[i].r;
        tep = pow(str[i].x, 2) + pow(str[i].y, 2);
        teq = pow(str[i].r, 2);
        if (tep >= teq) {
            str[i].flag = 0;
            cnt++;
        } else
            str[i].p = (str[i].r - sqrt(tep)) * 3.1415926 * str[i].r * str[i].r;
        }
    if (n - cnt <= k)
        cout << "0.0000000000000";
    else {
        sort(str, str + n, comp);
        for (int i = 0; i < n; i++) {
            if (str[i].flag) {
                tot += str[i].p;
                cnu++;
            }
            if (cnu == n - cnt - k)
                break;
        }
        printf("%.8lf", tot);
    }

    return 0;
}