【计算几何】【极角序】【前缀和】bzoj1132 [POI2008]Tro

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
using namespace std;
#define N 3002
typedef double db;
typedef long long ll;
struct Point{ll x,y;db jiao;}p
,v
;
int n;
ll sumx
,sumy
,ans;
bool cmp(const Point &a,const Point &b){return a.y<b.y;}
bool cmp2(const Point &a,const Point &b){return a.jiao<b.jiao;}
int main()
{
scanf("%d",&n);
for(int i=1;i<=n;++i)
scanf("%lld%lld",&p[i].x,&p[i].y);
sort(p+1,p+n+1,cmp);
for(int i=1;i<=n-2;++i)
{
for(int j=i+1;j<=n;++j)
{
v[j].x=p[j].x-p[i].x;
v[j].y=p[j].y-p[i].y;
v[j].jiao=atan2((db)v[j].y,(db)v[j].x);
}
sort(v+i+1,v+n+1,cmp2);
for(int j=n;j>i;--j)
{
sumx[j]=sumx[j+1]+v[j].x;
sumy[j]=sumy[j+1]+v[j].y;
}
for(int j=i+1;j<n;++j)
ans+=(v[j].x*sumy[j+1]-v[j].y*sumx[j+1]);
}
printf("%lld",ans>>1);
if(ans&1) puts(".5");
else puts(".0");
return 0;
}