hdu1392凸包裸题

//极角排序
#include <bits/stdc++.h>
#define sqr(x) ((x)*(x))
using namespace std;
int n,st[200001],top;
struct POI
{
    int x,y;
    POI()
    {
        x=y=0;
    }
    POI(int _x,int _y)
    {
        x=_x;y=_y;
    }
} p[200001];
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[1],y-p[1])>0 || cross(x-p[1],y-p[1])==0 && dis(x,p[1])<dis(y,p[1]));
}
int main()
{
    for(scanf("%d",&n);n;scanf("%d",&n))
    {
        for(int i=1;i<=n;i++)
            scanf("%d%d",&p[i].x,&p[i].y);
        if(n==2)//wtf 
        {
            printf("%.2f\n",dis(p[1],p[2]));
            continue;
        }
        int id=1;
        for(int i=2;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[1],p[id]);
        sort(p+2,p+n+1,com);
        st[top=1]=st[0]=1;
        for(int i=2;i<=n;i++)
        {
            while(top && cross(p[st[top]]-p[st[top-1]],p[i]-p[st[top]])<0) --top;
            st[++top]=i;
        }
        double len=0;
//        puts("");
//        for(int i=1;i<=top;i++)
//            printf("%d %d\n",p[st[i]].x,p[st[i]].y);
        st[++top]=1;
        for(int i=1;i<top;i++)
            len+=dis(p[st[i]],p[st[i+1]]);
        printf("%.2f\n",len);
    }
    return 0;
}