poj 2187 Beauty Contest Andrew

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
using namespace std;

struct Point
{
    int x,y;

} p[100000+5],ch[100000+5];

Point operator - (Point a,Point b)
{
    Point c;
    c.x=a.x-b.x;
    c.y=a.y-b.y;
    return c;
}

int cross(Point a,Point b)
{
    return a.x*b.y-b.x*a.y;
}

double dot(Point a,Point b)
{
    return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}

bool cmp(Point a,Point b)
{
    if(a.x==b.x) return a.y<b.y;
    else return a.x<b.x;
}

int main()
{
    int n,m,i,j,k;
    double sum,ans;
    scanf("%d",&n);
    for(i=0; i<n; i++)
        scanf("%d%d",&p[i].x,&p[i].y);
    sort(p,p+n,cmp);
    m=0;
    for(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];
    }
    k=m;
    for(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--;
    ans=0;
    for(i=0; i<m; i++)
        for(j=i+1; j<m; j++)
        {
            sum=dot(ch[i],ch[j]);
            if(sum>ans) ans=sum;
        }
    printf("%.0f\n",ans);
    return 0;
}