HDU ACM 1392 Surround the Trees->凸包

分析：直接求出凸包，再算边长即可。另外只有一个点时为0.00单独处理，两个点直接为距离也单独处理。

#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;

struct Point
{
	Point(){}
	Point(double _x,double _y):x(_x),y(_y){}
	Point operator-(const Point& a) const
	{
		return Point(x-a.x,y-a.y);
	}
	double x,y;
};

double dis(const Point& a,const Point& b)
{
	return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}

double cross(const Point& a,const Point& b)
{
	return a.x*b.y-a.y*b.x;
}

bool cmp(const Point& a,const Point& b)
{
	if(a.x!=b.x)
		return a.x<b.x;
	else
		return a.y<b.y;
}

int convexhull(Point* p,int n,Point* ch)
{
	int i,m,k;

	sort(p,p+n,cmp);
	m=0;
	for(i=0;i<n;i++)   //上凸包
	{
		while(m>1 && cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;
		ch[m++]=p[i];
	}
	k=m;
	for(i=n-2;i>=0;i--) //下凸包
	{
		while(m>k && cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;
		ch[m++]=p[i];
	}
	if(n>1) m--;
	return m;
}

int main()
{
	Point a[105],p[105];
	int n,i,m;
	double ans;

	while(scanf("%d",&n)==1 &&n)
	{
		for(i=0;i<n;i++)
			scanf("%lf%lf",&a[i].x,&a[i].y);
		if(n==1)
			printf("0.00\n");
		else if(n==2)
		{
			printf("%.2lf\n",dis(a[0],a[1]));
		}
		else
		{
			m=convexhull(a,n,p);
			ans=0;
			for(i=1;i<=m;i++)
				ans+=dis(p[i],p[i-1]);
			printf("%.2lf\n",ans);
		}
	}
	return 0;
}