【SCOI2007】bzoj1069 最大土地面积

#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const double eps=1e-8,pi=acos(-1);
int cmp(double x)
{
if (x>eps) return 1;
if (fabs(x)<=eps) return 0;
return -1;
}
struct Vector
{
double x,y;
bool operator < (const Vector &v) const
{
return cmp(x-v.x)==-1;
}
void rd()
{
scanf("%lf%lf",&x,&y);
}
Vector operator + (const Vector &v) const
{
return (Vector){x+v.x,y+v.y};
}
Vector operator - (const Vector &v) const
{
return (Vector){x-v.x,y-v.y};
}
}a[2010],f[2010];
typedef Vector Point;
double dot(Vector v,Vector u)
{
return v.x*u.x+v.y*u.y;
}
double cross(Vector v,Vector u)
{
return v.x*u.y-v.y*u.x;
}
double len(Vector v)
{
return sqrt(dot(v,v));
}
double size(Point a,Point b,Point c)
{
return fabs(cross(b-a,c-a))/2;
}
struct Line
{
Point p;
Vector v;
}l1;
double dis(Point p,Line l)
{
return fabs(cross(l.v,p-l.p))/len(l.v);
}
int n,m,mm;
int main()
{
double ans=0;
scanf("%d",&n);
for (int i=1;i<=n;i++) a[i].rd();
sort(a+1,a+n+1);
f[0]=a[1];
m=1;
for (int i=2;i<=n;i++)
{
while (m>1&&cmp(cross(f[m-1]-f[m-2],a[i]-f[m-2]))>=0) m--;
f[m++]=a[i];
}
mm=m;
for (int i=n-1;i;i--)
{
while (m>mm&&cmp(cross(f[m-1]-f[m-2],a[i]-f[m-2]))>=0) m--;
f[m++]=a[i];
}
m--;
for (int i=0;i<m;i++)
for (int j=(i-2+m)%m,x=(i-1+m)%m,nex=(x-1+m)%m,y=(j-1+m)%m,ney=(y-1+m)%m;
j!=(i+1)%m;j=(j-1+m)%m)
{
while (nex!=j&&cmp(size(f[i],f[j],f[nex])-size(f[i],f[j],f[x]))==1)
x=nex,nex=(x-1+m)%m;
while (ney!=j&&cmp(size(f[i],f[j],f[ney])-size(f[i],f[j],f[y]))==1)
y=ney,ney=(y-1+m)%m;
ans=max(ans,size(f[i],f[j],f[x])+size(f[i],f[j],f[y]));
}
printf("%.3f\n",ans);
}