poj 2187 求凸包最远点对--旋转卡壳

#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

const int maxn=100003;
struct point
{
int x,y;
}p[maxn],s[maxn];

int xmult(point p1,point p2,point p0)
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x);
}

int dist(point p1,point p2)
{
return (p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y);
}

int cmp(point a,point b)
{
return a.y<b.y || a.y==b.y && a.x<b.x;
}
int main()
{
freopen("pin.txt","r",stdin);
freopen("pou.txt","w",stdout);
int i,j,k,t,n,tmp,ans;
cin >> n;
for (i=0;i<n;i++)
cin >> p[i].x >> p[i].y;
sort(p,p+n,cmp);
s[0]=p[0];s[1]=p[1];
k=1;
for (i=2;i<n;i++)
{
while (k && xmult(p[i],s[k],s[k-1])>=0)
k--;
s[++k]=p[i];
}
tmp=k;
s[++k]=p[n-2];
for (i=n-3;i>=0;i--)
{
while (k>tmp && xmult(p[i],s[k],s[k-1])>=0)
k--;
s[++k]=p[i];
}
s[k]=p[0];
for (ans=0,t=1,i=0;i<k;i++)
{
while (xmult(s[i+1],s[t],s[i])<xmult(s[i+1],s[t+1],s[i]))
t=(t+1)%k;
int a1=dist(s[i],s[t]);
int a2=dist(s[i+1],s[t+1]);
if (a1>ans) ans=a1;
if (a2>ans) ans=a2;
}
cout << ans;
return 0;
}