bzoj1069 SCOI2007 最大土地面积

 1 /*by SilverN*/
 2 #include<iostream>
 3 #include<cstdio>
 4 #include<cmath>
 5 #include<cstring>
 6 #include<algorithm>
 7 #define ll long long
 8 using namespace std;
 9 const int mxn=3000;
10 struct P{
11     double x,y;
12 }p[mxn],s[mxn];
13 int top=0;
14 int n;
15 double ans=0;
16 //
17 inline P operator - (P a,P b){
18     P t; t.x=a.x-b.x; t.y=a.y-b.y; return t;
19 }
20 inline double operator * (P a,P b){
21     return (a.x*b.y)-(b.x*a.y);
22 }
23 inline double dis(P a,P b){
24     return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
25 }
26 inline bool operator < (P a,P b){
27     double t=(a-p[1])*(b-p[1]);
28     if(t==0)return dis(a,p[1])<dis(b,p[1]);
29     return t<0;
30 }
31 //
32 void graham(){
33     int t=1;
34     int i,j;
35     for(i=2;i<=n;i++){
36         if(p[i].y<p[t].y || (p[i].y==p[t].y && p[i].x<p[t].x))t=i;
37     }
38     swap(p[1],p[t]);
39     sort(p+2,p+n+1);
40     s[++top]=p[1];s[++top]=p[2];
41     for(i=3;i<=n;i++){
42         while(top>1 && (p[i]-s[top-1])*(s[top]-s[top-1])<=0)
43             top--;
44         s[++top]=p[i];
45     }
46     s[top+1]=p[1];
47     return;
48 }
49 double solve()
50 {
51     s[top+1]=p[1];
52     double ans=0;
53     int a,b;
54     for(int x=1;x<=top;x++)
55     {
56         a=x%top+1;b=(x+2)%top+1;
57         for(int y=x+2;y<=top;y++)
58         {
59             while(a%top+1!=y&&(s[y]-s[x])*(s[a+1]-s[x])>(s[y]-s[x])*(s[a]-s[x]))
60                 a=a%top+1;
61             while(b%top+1!=x&&(s[b+1]-s[x])*(s[y]-s[x])>(s[b]-s[x])*(s[y]-s[x]))
62                 b=b%top+1;
63             ans=max((s[y]-s[x])*(s[a]-s[x])+(s[b]-s[x])*(s[y]-s[x]),ans);//(s[b]-s[x])*(s[y]-s[x])前后颠倒成(s[y]-s[x])*(s[b]-s[x])就会WA，不能理解
64         }
65     }
66     return ans;
67 }
68 int main(){
69     scanf("%d",&n);
70     int i,j;
71     for(i=1;i<=n;i++){
72         scanf("%lf%lf",&p[i].x,&p[i].y);
73     }
74     graham();
75     printf("%.3lf\n",solve()/2);
76     return 0;
77 }