#include <iostream>
#include <algorithm>
#include <stdlib.h>
#define N 100
using namespace std;int n=0;
struct POINT
{int x,y;
}p[N],ans[N];
int visit[N],mark[N];
int Djudge(POINT a1,POINT a2,POINT a3)
{int calculate=a1.x*a2.y+a3.x*a1.y-a3.x*a2.y-a2.x*a1.y-a1.x*a3.y;return calculate;
}
bool cmpxy(const POINT a,const POINT b)
{if(a.x!=b.x)return a.x<b.x;elsereturn a.y<b.y;
}/*在涉及到平面上点对问题时，经常会按照这种方法对点进行排序
这与后面的sort(p,p+n,cmpxy)经常一起使用，在最近我做的2D maximal finding problem时，也是
使用了这样的排序对点对进行预处理。
*/
void DealLeft(int first,int last)
{int max=0,index=-1;int i=first;if(first<last){for(i=first+1;i<last;i++){int calcu=Djudge(p[first],p[i],p[last]);if(calcu==0)visit[i]=1;if(calcu>max){max=calcu;index=i;}}}else{for(i-1;i>last;i--){int calcu=Djudge(p[first],p[i],p[last]);if(calcu==0)visit[i]=1;if(calcu>max){max=calcu;index=i;}}}if(index!=-1){visit[index]=1;DealLeft(first,index);DealLeft(index,last);}
}
int main()
{cout<<"Enter the number of the points: ";cin>>n;cout<<"Enter the points: ";for(int i=0;i<n;i++){cin>>p[i].x>>p[i].y;visit[i]=0;}visit[0]=1;visit[n-1]=1;sort(p,p+n,cmpxy);DealLeft(0,n-1);DealLeft(n-1,0);int t=0;for(int i=0;i<n;i++){if(visit[i]==1){ans[t].x==p[i].x;ans[t].y==p[i].y;t++;}}
//顺时针输出mark[0]=mark[t-1]=1;for(int i=1;i<t-1;i++)mark[i]-0;cout<<ans[0].x<<" "<<ans[0].y<<endl;for(int i=1;i<t-1;i++){int d=Djudge(ans[0],ans[t-1],ans[i]);if(d>=0){cout<<ans[i].x<<" "<<ans[i].y<<endl;mark[i]=1;}}cout<<ans[t-1].x<<" "<<ans[t-1].y<<endl;for(int i=1;i<t;i++){if(mark[i]!=1){int d=Djudge(ans[0],ans[t-1],ans[i]);if(d<0){cout<<ans[i].x<<" "<<ans[i].y<<endl;}}}return 0;
}