poj 3348

裸的凸包，，我就来粘一下板子

把jls和dls的代码混合了一下(雾)，感觉我要是能把camp里的几何掌握的差不多就稳         了。(mxy到底打了什么?)

感觉叉积实在是个好东西qwq

今晚cf还要不要打呢，，，可是这几天都是十一点多就早睡了。。。到时候神智不清怎么办，，，可是好想上个紫什么的。。。

 #include <cstdio>
 #include <cmath>
 #include <algorithm>
 #include <vector>
 using namespace std;
 typedef double db;
 const db eps=1e-;
 const db pi=acos(-);
 int sign(db k){
     if (k>eps) return ; else if (k<-eps) return -; return ;
 }
 int cmp(db k1,db k2){return sign(k1-k2);}
 struct point{
     db x,y;
     point operator + (const point &k1) const{return (point){k1.x+x,k1.y+y};}
     point operator - (const point &k1) const{return (point){x-k1.x,y-k1.y};}
     point operator * (db k1) const{return (point){x*k1,y*k1};}
     point operator / (db k1) const{return (point){x/k1,y/k1};}
     bool operator == (const point &k1) const{return cmp(x,k1.x)==&&cmp(y,k1.y)==;}
     bool operator <(const point &k1)const {
         int c=cmp(x,k1.x);
         if(c)return c==-;
         return cmp(y,k1.y)==-;
     }
 };
 db cross(point k1,point k2){return k1.x*k2.y-k1.y*k2.x;}
 db dot(point k1,point k2){return k1.x*k2.x+k1.y*k2.y;}
 int n;
 vector<point> convexHull(vector<point> ps){
     int n = ps.size();if(n<=)return ps;
     sort(ps.begin(),ps.end());
     vector<point> qs(n*);int k=;
     for(int i=;i<n;qs[k++]=ps[i++])
         while (k>&&cross(qs[k-]-qs[k-],ps[i]-qs[k-])<=)--k;
     for(int i=n-,t=k;i>=;qs[k++]=ps[i--])
         while (k>t&&cross(qs[k-]-qs[k-],ps[i]-qs[k-])<=)--k;
     qs.resize(k-);
     return qs;
 }
 vector<point>p;
 db x,y;
 int main(){
     scanf("%d",&n);
     for(int i=;i<=n;i++){
         scanf("%lf%lf",&x,&y);
         p.push_back(point{x,y});
     }
     p=convexHull(p);
     int m=p.size();
     int ans=;
     for(int i=;i<m;i++){
         ans+=cross(p[i],p[(i+)%m]);
     }
     ans=abs(ans)/;
     printf("%d\n",ans/);
 }