poj3348 Cows 凸包 叉积求多边形面积

graham扫描法，参考yyb

#include <algorithm>
#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int n, din;
const double eps=1e-7;
struct Point{
	double x, y;
	Point(double u=0.0, double v=0.0){
		x = u; y = v;
	}
	double crs(Point u){
		return x*u.y-y*u.x;
	}
	Point operator-(const Point &u)const{
		return Point(x-u.x, y-u.y);
	}
	double norm(){
		return sqrt(x*x+y*y);
	}
}pt[10005], sta[10005];
bool cmp(Point u, Point v){
	double tmp=(u-pt[1]).crs(v-pt[1]);
	if(fabs(tmp)<eps)	return u.norm()<v.norm();
	else	return tmp>0;
}
int main(){
	cin>>n;
	for(int i=1; i<=n; i++)
		scanf("%lf %lf", &pt[i].x, &pt[i].y);
	int ps=1;
	for(int i=2; i<=n; i++)
		if(pt[i].x<pt[ps].x || (pt[i].x==pt[ps].x && pt[i].y<pt[ps].y))
			ps = i;
	swap(pt[1], pt[ps]);
	sort(pt+2, pt+1+n, cmp);
	sta[++din] = pt[1];
	for(int i=2; i<=n; i++){
		while(din>=2 && (pt[i]-sta[din-1]).crs(sta[din]-sta[din-1])>=-eps)
			din--;
		sta[++din] = pt[i];
	}
	sta[din+1] = sta[1];
	double ans=0;
	for(int i=1; i<=din; i++)
		ans += sta[i].crs(sta[i+1]);
	ans /= 2;
	printf("%d\n", (int)ans/50);
	return 0;
}