Cod sursă (job #173422)

#include <fstream>
#include <cmath>
#include <algorithm>
#include <iomanip>
#include <vector>


using namespace std;

ifstream cin ("infasuratoare.in");
ofstream cout ("infasuratoare.out");


/*struct pn{
       double x,y;
       }v[120010];*/

#define x first
#define y second

typedef pair <double,double> pn;
       
pn stk[120010],v[120010];;

double ccw(pn p1, pn p2, pn p3) // <0 dup acele cs // >0 impotriva acelor cs // =0 coliniare
{
       return( (p2.x - p1.x)*(p3.y - p1.y) - (p2.y - p1.y)*(p3.x - p1.x) );
       }
       
bool cmp(pn p1, pn p2){
     return (ccw(v[1],p1,p2)<0);
     }
     




int N, head;

///////////////////////////////////////////////////////////////////////////

int main() {
    
    int poz=1;
    
    cin >> N;
    for(int i(1); i<=N; ++i){
            cin >> v[i].x >> v[i].y;
         //   if( v[i].x<v[poz].x || (!(v[poz].x<v[i].x) && v[i].y<v[poz].y) ) poz=i;
         if(v[i]<v[poz]) poz=i;
            }
     
    swap(v[1],v[poz]);
    sort(v+2, v+N+1, cmp);
    
    stk[1]=v[1]; stk[2]=v[2];
    head=2;
    for(int i=3; i<=N; ++i) {
                             while( (head>=2) && (ccw(stk[head-1],stk[head],v[i])>0) ) --head;
                             stk[++head]=v[i]; 
                              }
    
  //  cout << head << "\n";
    cout << fixed;
    
    vector<pn> rep (stk+1, stk+head+1);
    
 //   for(int i=head-1; i>=0; i--) cout << rep[i].x << " " << rep[i].y << "\n";
    
    int i,j,k;
    i=head-1; j=head-2; k=head-3;
    
   /* while(i>-1) {
                     if( ccw(rep[i],rep[j],rep[k])==0 ) { rep.erase(rep.begin()+j); head--; cout << "sters\n";};
                     i--; j--; k--;
                     if(j==0) j=head-1;
                     if(k==0) k=head-1;
                     } */
    head--;                 
    for(int i=0; i<=head; i++) if( ccw(rep[i],rep[(i+1)%head],rep[(i+2)%head])==0 ) { rep.erase(rep.begin()+j); head--; cout << "sters\n";}
                     
   // cout << "\n\n\n\n";
    cout << head << "\n";

    for(int i=head-1; i>=0; i--) cout << rep[i].x << " " << rep[i].y << "\n";
    
 //   cout << ccw(rep[0], rep[head-1], rep[head-2]);

    
return 0;    
}