2015亚洲区北京站网络赛

A http://hihocoder.com/problemset/problem/1227

题意：给出平面上m个点，任选一个点为圆心，选最小的半径使得恰好n个点在圆内。

解法：枚举圆心，然后对所有点到该圆心距离排序，取第n大的距离作为半径，若第n+1大的在圆内或圆上不合法。

 1 //#define debug
 2 //#define txtout
 3 #include<cstdio>
 4 #include<cstdlib>
 5 #include<cstring>
 6 #include<cmath>
 7 #include<cctype>
 8 #include<ctime>
 9 #include<iostream>
10 #include<algorithm>
11 #include<vector>
12 #include<queue>
13 #include<stack>
14 #include<map>
15 #include<set>
16 #define mt(a,b) memset(a,b,sizeof(a))
17 using namespace std;
18 typedef long long LL;
19 const double eps=1e-8;
20 const double pi=acos(-1.0);
21 const int inf=0x3f3f3f3f;
22 const int M=1e2+10;
23 struct point {
24     double x,y;
25 } p[M];
26 double dist[M];
27 int n,m;
28
29 double Square(double x) { ///平方
30     return x*x;
31 }
32 double Distance(point a,point b) { ///平面两点距离
33     return sqrt(Square(a.x-b.x)+Square(a.y-b.y));
34 }
35
36 int choose(point c) {
37     for(int i=0; i<m; i++) {
38         dist[i]=Distance(c,p[i]);
39     }
40     sort(dist,dist+m);
41     double r=dist[n-1];
42     int int_r=(int)(r+1);
43     if(n<m&&dist[n]<int_r+eps)
44         return inf;
45     return int_r;
46 }
47 int solve() {
48     if(n>m) return -1;
49     int res=inf;
50     for(int i=0; i<m; i++) {
51         res=min(res,choose(p[i]));
52     }
53     if(res==inf) res=-1;
54     return res;
55 }
56 int main() {
57 #ifdef txtout
58     freopen("in.txt","r",stdin);
59     freopen("out.txt","w",stdout);
60 #endif
61     int t;
62     while(~scanf("%d",&t)) {
63         while(t--) {
64             scanf("%d%d",&m,&n);
65             for(int i=0; i<m; i++) {
66                 scanf("%lf%lf",&p[i].x,&p[i].y);
67             }
68             printf("%d\n",solve());
69         }
70     }
71     return 0;
72 }

end