POJ 2007

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;
const int MAXN=100;
int n,l;

int st[MAXN],stop,cnt;
int ans[MAXN];

struct point{
	int x,y;
}p[MAXN];

bool cmp(point A,point B){
	if(A.y<B.y) return true;
	else if(A.y==B.y){
		if(A.x<B.x) return true;
	}
	return false;
}

bool multi(point a,point b,point c){
	return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x)<=0;
}

void slove(){
	stop=0; cnt=0;
	st[stop++]=0; st[stop++]=1;
	for(int i=2;i<n;i++){
		while(stop>1&&multi(p[st[stop-1]],p[i],p[st[stop-2]])) stop--;
		st[stop++]=i;
	}
	for(int i=0;i<stop;i++){
		ans[cnt++]=st[i];
	}
	stop=0;
	st[stop++]=n-1; st[stop++]=n-2;
	for(int i=n-3;i>=0;i--){
		while(stop>1&&multi(p[st[stop-1]],p[i],p[st[stop-2]])) stop--;
		st[stop++]=i;
	}
	for(int i=1;i<stop-1;i++)
	ans[cnt++]=st[i];
}

int main(){
	n=0;
	while(scanf("%d%d",&p[n].x,&p[n].y)!=EOF) n++;
	sort(p,p+n,cmp);
	slove();
	int i,k;
	for(k=0;k<cnt;k++){
		if(p[ans[k]].x==0&&p[ans[k]].y==0) break;
	}
	printf("(%d,%d)\n",p[ans[k]].x,p[ans[k]].y);
	for(int i=k+1;i<cnt;i++){
		printf("(%d,%d)\n",p[ans[i]].x,p[ans[i]].y);
	}
	for(int i=0;i<k;i++){
		printf("(%d,%d)\n",p[ans[i]].x,p[ans[i]].y);
	}
	return 0;
}