Cod sursă (job #770906)

#include <iostream>
#include <fstream>
#include <iomanip>
#include <algorithm>
using namespace std;
ifstream fin("infasuratoare.in");
ofstream fout("infasuratoare.out");
int n;
struct abc{
   int x,y;
}a[100005];
int det(abc a,abc b,abc c)
{
    return (a.x*b.y+a.y*c.x+b.x*c.y-c.x*b.y-a.y*b.x-a.x*c.y);
}
int comp(abc b,abc c)
{
    return (det(a[1],b,c)>0);
}
int main()
{
    fin>>n;
    int mn=1;
    for (int i=1;i<=n;i++)
    {
        fin>>a[i].x>>a[i].y;
        if (a[i].x<a[mn].x || a[i].x==a[mn].x && a[i].y<a[mn].y) mn=i;
    }
    swap(a[1],a[mn]);
    sort (a+2,a+n+1, comp);
    int k=2;
    a[n+1]=a[1];
    for (int i=3;i<=n+1;i++)
    {
        while (k>=2 && det(a[k-1],a[k],a[i])<=0) k--;
        a[++k]=a[i];
    }
    fout<<k-1<<'\n';
    for (int i=2;i<=k;i++)
        fout<<a[i].x<<" "<<a[i].y<<'\n';
    return 0;
}