hdu1392凸包裸题

 //极角排序
 #include <bits/stdc++.h>
 #define sqr(x) ((x)*(x))
 using namespace std;
 int n,st[],top;
 struct POI
 {
     int x,y;
     POI()
     {
         x=y=;
     }
     POI(int _x,int _y)
     {
         x=_x;y=_y;
     }
 } p[];
 int cross(POI x,POI y)
 {
     return x.x*y.y-x.y*y.x;
 }
 double dis(POI x,POI y)
 {
     return sqrt(sqr(x.x-y.x)+sqr(x.y-y.y));
 }
 POI operator-(POI x,POI y)
 {
     return POI(x.x-y.x,x.y-y.y);
 }
 bool com(POI x,POI y)
 {
     return(cross(x-p[],y-p[])> || cross(x-p[],y-p[])== && dis(x,p[])<dis(y,p[]));
 }
 int main()
 {
     for(scanf("%d",&n);n;scanf("%d",&n))
     {
         for(int i=;i<=n;i++)
             scanf("%d%d",&p[i].x,&p[i].y);
         if(n==)//wtf
         {
             printf("%.2f\n",dis(p[],p[]));
             continue;
         }
         int id=;
         for(int i=;i<=n;i++)
             if(p[i].x<p[id].x || p[i].x==p[id].x && p[i].y<p[id].y) id=i;
         swap(p[],p[id]);
         sort(p+,p+n+,com);
         st[top=]=st[]=;
         for(int i=;i<=n;i++)
         {
             while(top && cross(p[st[top]]-p[st[top-]],p[i]-p[st[top]])<) --top;
             st[++top]=i;
         }
         double len=;
 //        puts("");
 //        for(int i=1;i<=top;i++)
 //            printf("%d %d\n",p[st[i]].x,p[st[i]].y);
         st[++top]=;
         for(int i=;i<top;i++)
             len+=dis(p[st[i]],p[st[i+]]);
         printf("%.2f\n",len);
     }
     return ;
 }