poj 2187 Beauty Contest

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

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
using namespace std;

const int maxn=50000;
struct point
{
    int x,y;
    bool operator < (const point &a)const
    {
        return (x<a.x)||(x==a.x&&y<a.y);
    }
};

point p[maxn],ch[maxn];

int sqr(int x)
{
    return x*x;
}

int det(point a,point b,point c)
{
    return((b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y));
}

int dis(point a,point b)
{
    return sqr(a.x-b.x)+sqr(a.y-b.y);
}

int convex_hull(point *p,int n,point *ch)
{
    sort(p,p+n);
    int m=0;
    for(int i=0; i<n; i++)
    {
        while(m>1&&det(ch[m-2],ch[m-1],p[i])<=0) m--;
        ch[m++]=p[i];
    }
    int k=m;
    for(int i=n-2; i>=0; i--)
    {
        while(m>k&&det(ch[m-2],ch[m-1],p[i])<=0) m--;
        ch[m++]=p[i];
    }
    if(n>1) m--;
    return m;
}

int dia_rotating_calipers(int n)
{
    int dia=0,q=1;
    for(int i=0; i<n; i++)
    {
        while(det(ch[i],ch[i+1],ch[q+1])>det(ch[i],ch[i+1],ch[q]))
        {
            q=(q+1)%n;
        }
        dia=max(dia,max(dis(ch[i],ch[q]),dis(ch[i+1],ch[q+1])));
    }
    return dia;
}

int main()
{
    int n,cn;
    scanf("%d",&n);
    for(int i=0; i<n; i++)
    {
        scanf("%d%d",&p[i].x,&p[i].y);
    }
    cn=convex_hull(p,n,ch);
    printf("%d
",dia_rotating_calipers(cn));
    return 0;
}