二维凸包模版——旋转卡壳法

#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>
const int MAXN = 109;
const double eps = 1e-6;
struct point{
double x,y;
}p[MAXN],h[MAXN];

inline double distance(const point &p1,const point &p2){
return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
}//   求两点之间的距离
inline double multiply(const point &sp,const point &ep,const point &op){
return ((sp.x-op.x)*(ep.y-op.y)-(ep.x-op.x)*(sp.y-op.y));
}//判断sp,ep，op是否满足左转
int cmp(const void *a,const void *b){//按极角排序
point *p1 = (point *)a;
point *p2 = (point *)b;
double t = multiply(*p2,*p1,p[0]);
if(t>eps) return 1;
else if(fabs(t)<=eps)
{
if(distance(p[0],*p1)>distance(p[0],*p2)) return 1;
else return -1;
}
else return -1;
}
void anglesort(point p[],int n){//找到最左下方的点
int i,k=0;
point temp;
for(i=1;i<n;i++)
if(p[i].x<p[k].x ||( (p[i].x==p[k].x) && (p[i].y<p[k].y)))
k=i;
temp=p[0],p[0]=p[k],p[k]=temp;
qsort(p+1,n-1,sizeof(point),cmp);
}
void Graham_scan(point p[],point ch[],int n,int &len){//建立凸包
int i,top=2;
anglesort(p,n);
if(n<3){
for(i=0,len=n;i<n;i++) ch[i]=p[i];
return;
}
ch[0]=p[0],ch[1]=p[1],ch[2]=p[2];
p
=p[0];
for(i=3;i<n;i++){
while(multiply(p[i],ch[top],ch[top-1])>=0) top--;
ch[++top]=p[i];
}
len=top+1;
}
int main(){
int i,j,n,len;
double d,ans;
scanf("%d",&n);
for(i=0;i<n;i++) scanf("%lf %lf",&p[i].x,&p[i].y);
Graham_scan(p,h,n,len);
double sum=0;
for(i=1;i<len;i++)
{
sum+=distance(h[i],h[i-1]);
}
printf("%.2lf\n",sum);
return 0;
}