POJ2187Beauty Contest 旋转卡壳

题目链接

http://poj.org/problem?id=2187

先求凸包 
再求凸多边形直径

旋转卡壳模板题

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<cmath>
 #include<iostream>
 #define N 50010
 using namespace std;
 int n,tp,ans;
 struct P{
     int x,y;
     bool operator < (const P &b)const{return x==b.x?y<b.y:x<b.x;}
     P operator - (const P &b)const{return (P){x-b.x,y-b.y};}
 }a[N],s[N];
 int dis(P a){return a.x*a.x+a.y*a.y;}
 int crs(P a,P b){return a.x*b.y-a.y*b.x;}
 void getbag(){
     sort(a+,a++n);
     for(int i=;i<=n;i++){
         while(tp>&&crs(a[i]-s[tp-],s[tp]-s[tp-])>=)--tp;
         s[++tp]=a[i];
     }
     int now=tp;
     for(int i=n-;i>=;i--){
         while(tp>now&&crs(a[i]-s[tp-],s[tp]-s[tp-])>=)--tp;
         s[++tp]=a[i];
     }
     if(n>)--tp;
 }
 void rotate(){
     int p=;s[tp+]=s[];
     for(int i=;i<=tp;i++){
         while(crs(s[i+]-s[i],s[p]-s[i])<crs(s[i+]-s[i],s[p+]-s[i]))p=p%tp+;
         ans=max(ans,max(dis(s[p]-s[i]),dis(s[p+]-s[i+])));
     }
 }
 int main(){
     scanf("%d",&n);
     for(int i=;i<=n;i++)
     scanf("%d%d",&a[i].x,&a[i].y);
     getbag();rotate();
     //for(int i=1;i<=tp;i++)printf("%d %d\n",s[i].x,s[i].y);
     printf("%d\n",ans);
     return ;
 }