poj 2187 Beauty Contest——旋转卡壳

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define db double
using namespace std;
const int N=5e4+5;
int n,sta[N],top,ans;
struct Node{
  int x,y;
  Node(int a=0,int b=0):x(a),y(b) {}
  bool operator< (const Node &b)const
  {return x<b.x||(x==b.x&&y<b.y);}
  Node operator- (const Node &b)const
  {return Node(x-b.x,y-b.y);}
}t[N],a[N];
int rdn()
{
  int ret=0;bool fx=1;char ch=getchar();
  while(ch>‘9‘||ch<‘0‘){if(ch==‘-‘)fx=0;ch=getchar();}
  while(ch>=‘0‘&&ch<=‘9‘)ret=ret*10+ch-‘0‘,ch=getchar();
  return fx?ret:-ret;
}
int Mx(int a,int b){return a>b?a:b;}
int cross(Node u,Node v){return u.x*v.y-u.y*v.x;}
int Sqr(int x){return x*x;}
int dist(Node u,Node v){return Sqr(u.x-v.x)+Sqr(u.y-v.y);}
void rtc()
{
  a[n+1]=a[1];
  for(int i=1,p=2;i<=n;i++)
    {
      while(cross(a[p+1]-a[i],a[p+1]-a[i+1])>cross(a[p]-a[i],a[p]-a[i+1]))//>
    {p++;if(p>n)p=1;}//for if only 2 ver!!!
      ans=Mx(ans,Mx(dist(a[p],a[i]),dist(a[p+1],a[i+1])));
      //      ans=Mx(ans,Mx(dist(a[p],a[i+1]),dist(a[p-1],a[i])));
    }
}
int main()
{
  int tot=rdn();
  for(int i=1;i<=tot;i++)t[i].x=rdn(),t[i].y=rdn();
  sort(t+1,t+tot+1);
  for(int i=1;i<=tot;i++)
    {
      while(top>1&&cross(t[i]-t[sta[top]],t[i]-t[sta[top-1]])>=0)top--;
      sta[++top]=i;
    }
  for(int i=tot-1,lm=top;i;i--)
    {
      while(top>lm&&cross(t[i]-t[sta[top]],t[i]-t[sta[top-1]])>=0)top--;///top>lm
      sta[++top]=i;
    }
  n=top-1;
  for(int i=1;i<=n;i++)a[i]=t[sta[i]];
  rtc(); printf("%d\n",ans);
  return 0;
}