[BZOJ1069][SCOI2007]最大土地面积 凸包+旋转卡壳

 1 #include<iostream>
 2 #include<cstring>
 3 #include<cstdio>
 4 #include<cstdlib>
 5 #include<cmath>
 6 #include<algorithm>
 7 #define eps 1e-9
 8 using namespace std;
 9 struct point {
10     double x,y;
11     bool operator <(const point t) const {
12         return fabs(t.x-x)>eps?x<t.x:y<t.y;
13     }
14 }p[2500],s[2500];
15 point operator -(point t1,point t2) {
16     return (point){t1.x-t2.x,t1.y-t2.y};
17 }
18 double operator *(point t1,point t2) {
19     return t1.x*t2.y-t2.x*t1.y;
20 }
21 int n,top;
22 void get() {
23     s[0]=p[1];
24     for(int i=2;i<=n;i++) {
25         while(top&&(s[top]-s[top-1])*(p[i]-s[top])>=0) top--;
26         s[++top]=p[i];
27     }
28     int tmp=top;
29     for(int i=n-1;i>=1;i--)  {
30         while(top>tmp&&(s[top]-s[top-1])*(p[i]-s[top])>=0) top--;
31         s[++top]=p[i];
32     }
33 }
34 double cnts(point a,point b,point c) {
35     return fabs((b-a)*(c-a))/2;
36 }
37 int main() {
38     double maxn=0;
39     scanf("%d",&n);
40     for(int i=1;i<=n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);
41     sort(p+1,p+n+1);
42     get();
43     for(int i=0;i<top;i++) {
44         int j=(i+2)%top,k=(i+1)%top,w=(j+1)%top;
45         while(cnts(s[i],s[j],s[k])<cnts(s[i],s[j],s[k+1])) k=(k+1)%top;
46         double ans1=cnts(s[i],s[j],s[k]);
47         while(cnts(s[i],s[j],s[w])<cnts(s[i],s[j],s[w+1])) w=(w+1)%top;
48         double ans2=cnts(s[i],s[j],s[w]);
49         double ans=0;
50         while(ans1+ans2>ans) {
51             ans=ans1+ans2;
52             j=(j+1)%top;
53             while(cnts(s[i],s[j],s[k])<cnts(s[i],s[j],s[k+1])) k=(k+1)%top;
54             ans1=cnts(s[i],s[j],s[k]);
55             while(cnts(s[i],s[j],s[w])<cnts(s[i],s[j],s[w+1])) w=(w+1)%top;
56             ans2=cnts(s[i],s[j],s[w]);
57         }
58         maxn=max(maxn,ans);
59     }
60     printf("%.3lf",maxn);
61 }