hdu 1392 Surround the Trees(凸包模板题)

hdu 1392 Surround the Trees

凸包模板直接套，注意n==2的时候只算一半

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
#define MAXN 105
struct point
{
int x,y;
bool operator < (const point &a) const
{
if(a.x==x) return y<a.y;
else return x<a.x;
}
};
struct vector
{
int x,y;
vector(int x,int y):x(x),y(y){}
};
vector operator - (point a,point b)
{
return vector(a.x-b.x,a.y-b.y);
}
int cross(vector a,vector b)
{
return a.x*b.y-a.y*b.x;
}
double getlen(point a,point b)
{
return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
int convex_hull(point *p,int n,point *ch)
{
sort(p,p+n);
int m=0;
for(int i=0;i<n;i++)
{
while(m>1&&cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<0) m--;
ch[m++]=p[i];
}
int k=m;
for(int i=n-2;i>=0;i--)
{
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 p[MAXN],ch[MAXN];
int n;
while(scanf("%d",&n)!=EOF&&n)
{
for(int i=0;i<n;i++)
scanf("%d%d",&p[i].x,&p[i].y);
int m=convex_hull(p, n, ch);
double ans=0;
for(int i=0;i<m;i++)
ans+=getlen(ch[i], ch[(i+1)%m]);
if(n==2) ans/=2.0;
printf("%.2lf\n",ans);
}
return 0;
}