凸包

一篇很不错的博客https://blog.csdn.net/bone_ace/article/details/46239187

贴个大佬博客找的板子

#include 
#include 
#include 
const int N=1e5+5;
const double eps=1e-6;
int n,top;
double len,S;
struct Point { double x,y; } a[N],b[N];
double dis(Point x,Point y)
{
	return sqrt((x.x-y.x)*(x.x-y.x)+(x.y-y.y)*(x.y-y.y));
}
double Cross(Point x,Point y,Point z)
{
	return (x.x-z.x)*(y.y-z.y)-(x.y-z.y)*(y.x-z.x);
}
double Cross(Point x,Point y)
{
	return x.x*y.y-x.y*y.x;
}
bool cmp(const Point &x,const Point &y)
{
	double t=Cross(x,y,a[1]);return abs(t)>eps?t>0:dis(a[1],x)1&&Cross(a[i],b[top],b[top-1])>=0) top--;
		b[++top]=a[i];
	}
	b[top+1]=b[1];
}
int main()
{
	scanf("%d",&n);
	for(int i=1;i<=n;i++) scanf("%lf%lf",&a[i].x,&a[i].y);
	Graham();//求凸包的点 
	for(int i=1;i<=top;i++) len+=dis(b[i],b[i+1]);
	printf("L: %.8lf\n",len);//求凸包的长度 
	for(int i=1;i<=top;i++) S+=Cross(b[i],b[i+1]);
	printf("S: %.8lf\n",S/2);//求凸包的面积 
}