Beauty Contest(凸包求最远点)

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

题意：求凸包上最远点距离的平方

思路：开始用旋转卡壳求的最远点，WA，改了好久。。后来又改成枚举凸包上的点。。AC了。。

#include <stdio.h>
#include <algorithm>
#include <math.h>
using namespace std;
const int N=50005;
int n;
struct Point
{
    int x,y;
    Point(int x = 0,int y = 0):x(x),y(y) {}
    bool friend operator < (const Point &a,const Point &b)
    {
        return a.x < b.x||(a.x==b.x&&a.y < b.y);
    }

} p[N],ch[N];
typedef Point Vector;
Vector operator-(Point a,Point b)
{
    return Vector(a.x-b.x,a.y-b.y);
}
int Cross(Vector A,Vector B)
{
    return A.x*B.y-A.y*B.x;
}
int Distance(Point a,Point b)
{
    return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}
int Graham()//判断凸包
{
    sort(p,p+n);
    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--;
    return m;
}
int main()
{
    scanf("%d",&n);
    for (int i = 0; i < n; i++)
        scanf("%d %d",&p[i].x,&p[i].y);
    int cnt = Graham();
    int ans = 0;
    for (int i = 0; i < cnt ; i++)
    {
        for (int j = 0; j < i; j++)
        {
            int dis = Distance(ch[i],ch[j]);
            ans = max(ans,dis);
        }
    }
    printf("%d
",ans);
    return 0;
}

旋转卡壳的那段代码，不知道哪错了，先保留着吧。。后期再改。。

int dia_rotating_calipers(int cnt)//旋转卡壳
{
    int dia = 0;
    int  q = 1;
    for (int i = 0; i < cnt; i++)
    {
        while(Cross(ch[i+1]-ch[i],ch[q+1]-ch[i]) > Cross(ch[i+1]-ch[i],ch[q]-ch[i]))
            q = (q+1)%n;
        dia = max(dia,max(Distance(ch[i],ch[q]),Distance(ch[i+1],ch[q+1])));
    }
    return dia;
}