给出一些正方形，让你求这些正方形顶点之间的最大距离的平方。
//返回点集直径的平方
int diameter2(vector<Point> & points) {
vector<Point> p = ConvexHull(points);
int n = p.size();
if(n==1) return 0;
if(n==2) return Dist2(p[0], p[1]);
p.push_back(p[0]);
int ans = 0;
for(int u = 0, v = 1; u < n; ++u) {
for(;;) {
int diff = Cross(p[u+1]-p[u], p[v+1]-p[v]);
if(diff<=0) {
ans = max(ans, Dist2(p[u], p[v]));
if(diff==0) ans = max(ans, Dist2(p[u], p[v+1]));
break;
}
v = (v+1) % n;
}
}
return ans;
}