Cod sursă (job #823313)

#include<fstream>
#include<bitset>
#include<algorithm>
using namespace std;
ifstream cin("infasuratoare.in");
ofstream cout("infasuratoare.out");
#define P pair<int,int>
#define x first
#define y second
const int N=120001;
const double E=1e-12;
P v[N];
bitset<N> u;
int a[N],m;
long long A(P o,P a,P b)
{
    return 1LL*(a.x-o.x)*(b.y-o.y)-1LL*(b.x-o.x)*(a.y-o.y);
}
bool B(P a,P b)
{
    return a.y<b.y||(a.y==b.y&&a.x<b.x);
}
int main()
{
    int m,n;
    cin>>n;
    for(int i=1;i<=n;cin>>v[i].x>>v[i].y,++i);
    sort(v+1,v+n+1,B),a[1]=1,a[2]=2,m=2,u[2]=1;
    for(int i=1,p=1;i>0;i+=(p=(i==n?-p:p)))
        if(!u[i]) {
            for(;m>1&&A(v[a[m-1]],v[a[m]],v[i])<E;u[a[m--]]=0);
            a[++m]=i,u[i]=1;
        }
    cout<<m-1<<'\n';
    for(int i=1;i<m;cout<<v[a[i]].x<<' '<<v[a[i]].y<<'\n',++i);
    return 0;
}