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;}