学步园

照着大白书敲了凸包模板，然后暴力水过。

注意枚举凸包顶点的时候，因为距离是对称的，所以j的内循环从i+1开始。

#include<iostream>
#include<stdio.h>
#include<cstdio>
#include<stdlib.h>
#include<vector>
#include<string>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include <ctype.h>
using namespace std;
//poj 2187

int n;
struct Point
{
    int x;
    int y;
    Point(int x=0,int y=0):x(x),y(y){}
};
Point p[50010];
Point ch[50010];
double Cross(Point A,Point B)
{
    return A.x*B.y-A.y*B.x;
}
bool cmp(Point a,Point b)
{
    if(a.x==b.x)
    {
        return a.y<b.y;
    }
    return a.x<b.x;
}
Point operator-(Point a,Point b)
{
    return Point(a.x-b.x,a.y-b.y);
}
int ConvexHull(int n,Point*ch)
{
    sort(p,p+n,cmp);
    int m=0;
    for(int i=0;i<n;i++)
    {
        while(m>1&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;
        ch[m++]=p[i];
    }
    int k=m;
    for(int i=n-2;i>=0;i--)
    {
        while(m>k&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;
        ch[m++]=p[i];
    }
    if(n>1) m--;
    //for(int i=0;i<m;i++) cout<<ch[i].x<<" "<<ch[i].y<<endl;
    return m;
}
int main()
{
    freopen("input.txt","r",stdin);
    // freopen("data.txt","r",stdin);
     //freopen("out1.txt","w",stdout);
     scanf("%d",&n);
     for(int i=0;i<n;i++)
     {
         scanf("%d %d",&p[i].x,&p[i].y);
     }
  //   for(int i=0;i<n;i++) cout<<p[i].x<<" "<<p[i].y<<endl;
     int m=ConvexHull(n,ch);
   //  for(int i=0;i<m;i++) cout<<ch[i].x<<" "<<ch[i].y<<endl;
     int dis=0;
     for(int i=0;i<m;i++)
     {
         for(int j=0;j<m;j++)
         {
             if(i==j) continue;
             int d=(ch[i].x-ch[j].x)*(ch[i].x-ch[j].x)+(ch[i].y-ch[j].y)*(ch[i].y-ch[j].y);
             //cout<<d<<endl;
             dis=max(dis,d);
         }
     }
     printf("%d\n",dis);
     return 0;
}