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-6;
const db pi=acos(-1);
int sign(db k){
    if (k>eps) return 1; else if (k<-eps) return -1; return 0;
}
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)==0&&cmp(y,k1.y)==0;}
    bool operator <(const point &k1)const {
        int c=cmp(x,k1.x);
        if(c)return c==-1;
        return cmp(y,k1.y)==-1;
    }
};
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<=1)return ps;
    sort(ps.begin(),ps.end());
    vector<point> qs(n*2);int k=0;
    for(int i=0;i<n;qs[k++]=ps[i++])
        while (k>1&&cross(qs[k-1]-qs[k-2],ps[i]-qs[k-2])<=0)--k;
    for(int i=n-2,t=k;i>=0;qs[k++]=ps[i--])
        while (k>t&&cross(qs[k-1]-qs[k-2],ps[i]-qs[k-2])<=0)--k;
    qs.resize(k-1);
    return qs;
}
vector<point>p;
db x,y;
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%lf%lf",&x,&y);
        p.push_back(point{x,y});
    }
    p=convexHull(p);
    int m=p.size();
    int ans=0;
    for(int i=0;i<m;i++){
        ans+=cross(p[i],p[(i+1)%m]);
    }
    ans=abs(ans)/2;
    printf("%d
",ans/50);
}