poj 3348 cows 凸包面积

/**
大意： 给定多个点求其确定的面积可以存放多少个面积为50的东西
思路： 1、凸包确定多边形
          2、 多边形求面积
          3、 面积/50 取整
**/
#include <iostream>
#include <algorithm>
using namespace std;
#define maxn 10010
struct point {
    double x,y;
    point (double x=0,double y=0):x(x),y(y){}
};

point p[maxn],ch[maxn];
typedef point Vector;

Vector operator -(point a,point b){
    return Vector (a.x-b.x,a.y-b.y);
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}
double polygonarea(point *p,int n){
    double area =0;
    for(int i=1;i<n-1;i++){
        area += cross(p[i]-p[0],p[i+1]-p[0]);
    }
    return area/2;
}
bool cmp(point a,point b){
    if(a.x==b.x)
        return a.y<b.y;
    return a.x<b.x;
}

int convexHull(point *p,int n,point *ch){
    sort(p,p+n,cmp);
    int m=0;
    for(int i=0;i<n;i++){
        while(m>1&&cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0)
            m--;
        ch[m++] = p[i];
    }
    int k = m;
    for(int i= n-2;i>=0;i--){
        while(m>k&&cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0)
            m--;
        ch[m++] = p[i];
    }
    if(n>1) m--;
    return m;
}
int main()
{
    int n;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>p[i].x>>p[i].y;
    }
    int res = convexHull(p,n,ch);
    double resarea = polygonarea(ch,res);
    cout<<(int )resarea/50<<endl;
    return 0;
}