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+1,a+1+n);
    for(int i=1;i<=n;i++){
        while(tp>1&&crs(a[i]-s[tp-1],s[tp]-s[tp-1])>=0)--tp;
        s[++tp]=a[i];
    }
    int now=tp;
    for(int i=n-1;i>=1;i--){
        while(tp>now&&crs(a[i]-s[tp-1],s[tp]-s[tp-1])>=0)--tp;
        s[++tp]=a[i];
    }
    if(n>1)--tp;
}
void rotate(){
    int p=2;s[tp+1]=s[1];
    for(int i=1;i<=tp;i++){
        while(crs(s[i+1]-s[i],s[p]-s[i])<crs(s[i+1]-s[i],s[p+1]-s[i]))p=p%tp+1;
        ans=max(ans,max(dis(s[p]-s[i]),dis(s[p+1]-s[i+1])));
    }
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    scanf("%d%d",&a[i].x,&a[i].y);
    getbag();rotate();
    //for(int i=1;i<=tp;i++)printf("%d %d
",s[i].x,s[i].y);
    printf("%d
",ans);
    return 0;
}