HDU 1392 Surround the Trees（计算几何--凸包）

#include <iostream>
#include <algorithm>
#include <cmath>
#define ll long long 
using namespace std;
const ll inf=6666666;
struct NODE{
	int x,y;
};
NODE tr[105],l[105];
int t,top;
bool cmp(NODE a,NODE b)
{
		double A=atan2(a.y-tr[1].y,a.x-tr[1].x);
		double B=atan2(b.y-tr[1].y,b.x-tr[1].x);
		if(A!=B) return A<B;
		else return a.x<b.x;
}
int Cross(NODE a,NODE b,NODE c)
{
	return (a.x-b.x)*(b.y-c.y)-(a.y-b.y)*(b.x-c.x);
}
void Get()
{
	int k;
	tr[0]=NODE{inf,inf};
	for(int i=1;i<=t;i++)
	    if(tr[0].y>tr[i].y||(tr[0].y==tr[i].y&&tr[0].x>tr[i].x))
	    {
	    	tr[0]=tr[i];
	    	k=i;
		}
	swap(tr[k],tr[1]);
	sort(tr+2,tr+t+1,cmp);
	l[0]=tr[1]; l[1]=tr[2];  top=1;
	for(int i=3;i<=t;)
	{
		if(top&&(Cross(l[top-1],tr[i],l[top])>=0)) top--;
		else l[++top]=tr[i++];
	}
}	
inline double cal(NODE f,NODE g)
{
	return sqrt(1ll*(f.x-g.x)*(f.x-g.x)+1ll*(f.y-g.y)*(f.y-g.y));
}
inline void Getans()
{
	double ans=0;
	if(top==0)
	ans=0;
	else if(top==1)
	      ans+=cal(l[0],l[1]);
	      else {
	      	     l[++top]=l[0];
	      	     for(int i=0;i<top;i++)
	      	         {
	      	         	ans+=cal(l[i],l[i+1]);
					   }
		        }
		printf("%.2lf\n",ans);
} 
int main()
{
	while(scanf("%d",&t)&&t)
	{
		for(int i=1;i<=t;i++)
		{
			scanf("%d %d",&tr[i].x,&tr[i].y);
		}
		Get(); 
		Getans();
	}
	
  return 0;	
}