hdu Surround the Trees

题目链接：戳我

凸包模板

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define MAXN 100010
using namespace std;
int n,top;
double ans;
struct Node{int x,y;}t[MAXN],s[MAXN];
inline bool cmp(struct Node a,struct Node b)
{
    double A=atan2((a.y-t[1].y),(a.x-t[1].x));
    double B=atan2((b.y-t[1].y),(b.x-t[1].x));
    if(A!=B) return A<B;
    else return a.x<b.x;
}
inline double cross(Node a,Node b,Node c){return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x);}
inline void solve()
{
    t[0]=(Node){0x3f3f3f3f,0x3f3f3f3f};
    int k=0;
    for(int i=1;i<=n;i++)
        if(t[i].y<t[0].y||(t[i].y==t[0].y&&t[i].x<t[0].x))
            t[0]=t[i],k=i;
    swap(t[1],t[k]);
    sort(&t[2],&t[1+n],cmp);
    s[0]=t[1],s[1]=t[2];
    top=1;
    for(int i=3;i<=n;i++)
    {
        while(top&&cross(s[top-1],t[i],s[top])>=0.0) top--;
        s[++top]=t[i];
    }
}
int main()
{
    #ifndef ONLINE_JUDGE
    freopen("ce.in","r",stdin);
    #endif
    for(;;)
    {
        ans=0.0;
        scanf("%d",&n);
        if(!n) break;
        for(int i=1;i<=n;i++)
            scanf("%d%d",&t[i].x,&t[i].y);
        solve();
        if(top==0)
            ans=0;
        else if(top==1)
            ans=sqrt((s[0].x-s[1].x)*(s[0].x-s[1].x)+(s[0].y-s[1].y)*(s[0].y-s[1].y));  
        else
        {
            s[++top]=s[0];
            for(int i=0;i<top;i++)
            {
                Node a=s[i],b=s[i+1];
                ans+=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
            }
        }
        printf("%.2lf\n",ans);
    }
    return 0;
}