凸包

模板:

#include<bits/stdc++.h>
#define inf 1000000000
#define ll long long
using namespace std;

int n,top;
double ans;
struct node
{
    int x,y;
}p[5005],s[5005];

node operator-(node a,node b)
{
	node t;t.x=a.x-b.x;t.y=a.y-b.y;return t;
}

ll operator*(node a,node b)
{
	return a.x*b.y-a.y*b.x;
}

ll dis(node a,node b)
{
	return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}

bool operator<(node a,node b)
{
	ll t=(a-p[1])*(b-p[1]);
	if(t==0)return dis(p[1],a)<dis(p[1],b);
	return t>0;
}

double area(node a,node b,node c)
{
    return fabs(a.x*b.y+a.y*c.x+b.x*c.y-b.y*c.x-c.y*a.x-a.y*b.x)/2;
}

void graham()
{
	int t=1;
	for(int i=2;i<=n;i++)
		if(p[i].y<p[t].y||(p[i].y==p[t].y&&p[i].x<p[t].x))t=i;
	swap(p[1],p[t]);
	sort(p+2,p+n+1);
	s[++top]=p[1];s[++top]=p[2];
	for(int i=3;i<=n;i++)
	{
		while((s[top]-s[top-1])*(p[i]-s[top-1])<=0)top--;
		s[++top]=p[i];
	}
	//for(int i=1;i<=top;i++)//顺时针输出,弄一个栈可以逆序输出
    //    printf("(%d %d)\n",s[i].x,s[i].y);

}

double getarea()///计算面积
{
    double Area=0;
    for(int i=2;i<=top-1;i++)
        Area+=area(s[1],s[i],s[i+1]);
    return Area;
}

double getl()///计算周长
{
    s[top+1]=p[1];//连一个原点连一个环
	for(int i=1;i<=top;i++)
		ans+=sqrt(dis(s[i],s[i+1]));
    printf("%.2f\n",ans);
}

int main()
{
    scanf("%d",&n);
	for(int i=1;i<=n;i++)
        scanf("%d%d",&p[i].x,&p[i].y);

	graham();
	return 0;
}