hdu Surround the Trees

题目链接：戳我

凸包模板

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define MAXN 100010
using namespace std;
int n,top;
double ans;
struct Node{int x,y;}t[MAXN],s[MAXN];
inline bool cmp(struct Node a,struct Node b)
{
	double A=atan2((a.y-t[1].y),(a.x-t[1].x));
	double B=atan2((b.y-t[1].y),(b.x-t[1].x));
	if(A!=B) return A<B;
	else return a.x<b.x;
}
inline double cross(Node a,Node b,Node c){return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x);}
inline void solve()
{
	t[0]=(Node){0x3f3f3f3f,0x3f3f3f3f};
	int k=0;
	for(int i=1;i<=n;i++)
		if(t[i].y<t[0].y||(t[i].y==t[0].y&&t[i].x<t[0].x))
			t[0]=t[i],k=i;
	swap(t[1],t[k]);
	sort(&t[2],&t[1+n],cmp);
	s[0]=t[1],s[1]=t[2];
	top=1;
	for(int i=3;i<=n;i++)
	{
		while(top&&cross(s[top-1],t[i],s[top])>=0.0) top--;
		s[++top]=t[i];
	}
}
int main()
{
	#ifndef ONLINE_JUDGE
	freopen("ce.in","r",stdin);
	#endif
	for(;;)
	{
		ans=0.0;
		scanf("%d",&n);
		if(!n) break;
		for(int i=1;i<=n;i++)
			scanf("%d%d",&t[i].x,&t[i].y);
		solve();
		if(top==0)
			ans=0;
		else if(top==1)
			ans=sqrt((s[0].x-s[1].x)*(s[0].x-s[1].x)+(s[0].y-s[1].y)*(s[0].y-s[1].y));	
		else
		{
			s[++top]=s[0];
			for(int i=0;i<top;i++)
			{
				Node a=s[i],b=s[i+1];
				ans+=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
			}
		}
		printf("%.2lf\n",ans);
	}
	return 0;
}

叉积顺时针计算角度，如果算出来大于0，就会构造出不合法的凸包，否则就是合法的qwq。