bzoj 1069: [SCOI2007]最大土地面积

求出凸包，枚举一下对角线，然后就可以用神奇的旋转卡壳了（其实感觉旋转卡壳就是用凸包的性质给暴力加了一点小优化）

#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
int n,top;
double ans;
struct point{double x,y;}p[2005],s[2005];
double dis(point a, point b)
{
    return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y); 
}
point operator - (point a, point b)
{
    point t; t.x=a.x-b.x; t.y=a.y-b.y; return t;
}
double operator * (point a, point b)
{
    return a.x*b.y-a.y*b.x;
}
bool operator < (point a, point b)
{
    if ((a-p[1])*(b-p[1])==0) return dis(a,p[1])<dis(b,p[1]);
    return (a-p[1])*(b-p[1])>0;
}
void Graham()
{
    int k=1; 
    for (int i=2; i<=n; i++) 
        if (p[k].y>p[i].y || (p[k].y==p[i].y && p[k].x>p[i].x)) k=i;
    swap(p[1],p[k]); sort(p+2,p+n+1);
    s[++top]=p[1]; s[++top]=p[2];
    for (int i=3; i<=n; i++)
    {
        while (top>1 && (s[top]-p[i])*(s[top-1]-p[i])>=0) top--;
        s[++top]=p[i];
    }
}
void RC()
{
    s[top+1]=p[1];
    int a,b;
    for (int x=1; x<=top; x++)
    {
        a=x%top+1; b=(x+2)%top+1;
        for (int y=x+2; y<=top; y++)
        {
            while (a%top+1!=y && (s[y]-s[x])*(s[a+1]-s[x])<(s[y]-s[x])*(s[a]-s[x])) a=a%top+1;
            while (b%top+1!=x && (s[b+1]-s[x])*(s[y]-s[x])<(s[b]-s[x])*(s[y]-s[x])) b=b%top+1;
            ans=max(ans,-((s[y]-s[x])*(s[a]-s[x])+(s[b]-s[x])*(s[y]-s[x])));
        }
    }
}
int main()
{
    scanf("%d",&n);
    for (int i=1; i<=n; i++) scanf("%lf%lf",&p[i].x,&p[i].y);
    Graham(); RC();
    printf("%.3lf",ans/2);
    return 0;
}