POJ 1981 最大点覆盖问题(极角排序）

 1 //#include"bits/stdc++.h"
 2 #include<sstream>
 3 #include<iomanip>
 4 #include"cstdio"
 5 #include"map"
 6 #include"set"
 7 #include"cmath"
 8 #include"queue"
 9 #include"vector"
10 #include"string"
11 #include"cstring"
12 #include"time.h"
13 #include"iostream"
14 #include"stdlib.h"
15 #include"algorithm"
16 #define db double
17 #define ll long long
18 #define vec vectr<ll>
19 #define mt  vectr<vec>
20 #define ci(x) scanf("%d",&x)
21 #define cd(x) scanf("%lf",&x)
22 #define cl(x) scanf("%lld",&x)
23 #define pi(x) printf("%d\n",x)
24 #define pd(x) printf("%f\n",x)
25 #define pl(x) printf("%lld\n",x)
26 //#define rep(i, x, y) for(int i=x;i<=y;i++)
27 #define rep(i, n) for(int i=0;i<n;i++)
28 const int N   = 1e4+ 5;
29 const int mod = 1e9 + 7;
30 const int MOD = mod - 1;
31 const int inf = 0x3f3f3f3f;
32 const db  PI  = acos(-1.0);
33 const db  eps = 1e-10;
34 using namespace std;
35 struct P
36 {
37     db x,y;
38     db ang;
39     bool in;
40 };
41 P a[N],b[N];
42 db dis(P a,P b){
43     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
44 }
45 int cmp(P a,P b){
46     if(a.ang==b.ang) return a.in>b.in;//上端点在前
47     return a.ang>b.ang;
48 }
49 int main()
50 {
51     int n;
52     while(scanf("%d",&n)==1,n)
53     {
54         int ans=1;
55         for(int i=1;i<=n;i++) cd(a[i].x),cd(a[i].y);
56         for(int i=1;i<=n;i++)
57         {
58             int p=0;
59             for(int j=1;j<=n;j++){
60                 if(i==j||dis(a[i],a[j])>2.0+eps) continue;
61                 db ang=atan2(a[i].x-a[j].x,a[i].y-a[j].y);//i于j的极角
62                 db tha=acos(dis(a[i],a[j])/2.0);//极角波动范围
63                 b[p].ang=ang+tha+2*PI,b[p++].in=1;//上端点
64                 b[p].ang=ang-tha+2*PI,b[p++].in=0;//下端点
65             }
66             sort(b,b+p,cmp);
67             int tmp=1;
68             for(int j=0;j<p;j++){
69                 if(b[j].in==1) tmp++;
70                 else tmp--;
71                 ans=max(tmp,ans);
72             }
73         }
74         pi(ans);
75     }
76     return 0;
77 }