poj2187Beauty Contest 凸包最大距离

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;
const int N=60005;
inline int dcmp(int x){
	if(!x)return 0;
	return x>0?1:-1;
}
struct pt{
	int x,y;
}p[N],ans[N];
int top,n;
int s[N];
inline int dis(pt a,pt b){
	return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}
inline int cross(pt a,pt b,pt c){
	return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
}
bool cmp(pt a,pt b){
	int ans=cross(p[0],a,b);
	if(ans >0 || (!ans && dis(p[0],a)>dis(p[0],b)))return true;
	return false;
}
void graham(){
    s[0]=0;s[1]=1;top=1;
    for(int i=2;i!=n;i++){
        while(top && cross(p[s[top-1]],p[s[top]],p[i])<0)top--;
        s[++top]=i;
    }
    top++;
}
inline int rotating_calipers(){
	int q=1;s[top]=s[0];q=1;
	int ans=dis(p[s[0]],p[s[1]]);
	for(int i=0;i<top;i++){
		while(abs(cross(p[s[i+1]],p[s[i]],p[s[q+1]]))>abs(cross(p[s[i+1]],p[s[i]],p[s[q]])))
			q=(q+1)%top;
		ans=max(ans,max(dis(p[s[i]],p[s[q]]),dis(p[s[(i+1)%top]],p[s[q]])));
	}
	return ans;
}
int main(){
	int i,x=0;
    x=0;
    memset(s,0,sizeof(s));
    cin>>n;
	for(i=0;i<n;i++){
		scanf("%d%d",&p[i].x,&p[i].y);
        if(p[x].y>p[i].y || (p[x].y == p[i].y && p[x].x>p[i].x))x=i;
    }if(x)
        swap(p[0],p[x]);
	sort(p+1,p+n,cmp);
	graham();
	cout<<rotating_calipers()<<endl;
	return 0;
}