USACO FC，二维凸包

#include
#include
#include
#include
const int maxn=10000+10;
using namespace std;
struct node{double x,y;};
node p[maxn],z[maxn];
bool cmp(node u,node v){return u.x>n;
	for(i=1;i<=n;i++)scanf("%lf%lf",&p[i].x,&p[i].y);
	sort(p+1,p+n+1,cmp);
	z[1]=p[1];z[2]=p[2];
	int top=2;
	for(i=3;i<=n;i++){
		while(top>1 && cmpxl(p[i],z[top],z[top-1]))top--;
		z[++top]=p[i];
	}
	double ans=0;
	for(i=1;i=1;i--){
		while(top>1 && cmpxl(p[i],z[top],z[top-1]))top--;
		z[++top]=p[i];
	}	
	for(i=1;i