UVALive 4356 Fire-Control System——思维

极角排序，枚举半径，然后扫一圈、

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e4;
const double pi = acos(-1.0);
struct Point {
    int x, y, dis;
    double angle;
    bool operator < (const Point &rhs) const {
        return angle < rhs.angle;
    }
}p[maxn], t[maxn];
int main() {
    int n, k, ks = 0;
    while (~scanf("%d%d", &n, &k) && n+k) {
        for (int i = 1; i <= n; i++) {
            scanf("%d%d", &p[i].x, &p[i].y);
            p[i].dis = p[i].x*p[i].x + p[i].y*p[i].y;
            p[i].angle = atan2(p[i].y, p[i].x);
        }
        if (k == 0) {
            printf("Case #%d: %.2lf\n", ++ks, 0.0);
            continue;
        }
        sort(p+1, p+1+n);
        double ans = 1e9;
        for (int i = 1; i <= n; i++) {
            int cnt = 0;
            for (int j = 1; j <= n; j++) {
                if (p[j].dis <= p[i].dis) t[++cnt] = p[j];
            }
            double angle = 1e9;
            if (cnt < k) continue;
            for (int l = 1, r = k; l <= cnt; l++, r++) {
                if (r <= cnt) {
                    angle = min(angle, t[r].angle - t[l].angle);
                }
                else {
                    angle = min(angle, t[r-cnt].angle+2.0*pi-t[l].angle);
                }
            }
            ans = min(ans, 0.5*angle*p[i].dis);
        }
        printf("Case #%d: %.2lf\n", ++ks, ans);
    }
    return 0;
}