Cod sursă (job #644193)

#include <bits/stdc++.h>
using namespace std;

ifstream fin("infasuratoare.in");
ofstream fout("infasuratoare.out");

struct Point
{
    int x, y;
};

Point p0;

Point nextToTop(stack<Point> &st)
{
    Point p = st.top();
    st.pop();
    Point res = st.top();
    st.push(p);
    return res;
}

void swap(Point &p1, Point &p2)
{
    Point temp = p1;
    p1 = p2;
    p2 = temp;
}

int distSq(Point p1, Point p2)
{
    return (p1.x - p2.x)*(p1.x - p2.x) +
          (p1.y - p2.y)*(p1.y - p2.y);
}


int orientation(Point p, Point q, Point r)
{
    int val = (q.y - p.y) * (r.x - q.x) -
              (q.x - p.x) * (r.y - q.y);

    if (val == 0) return 0;
    return (val > 0)? 1: 2;
}

int compare(const void *vp1, const void *vp2)
{
   Point *p1 = (Point *)vp1;
   Point *p2 = (Point *)vp2;

   int o = orientation(p0, *p1, *p2);
   if (o == 0)
     return (distSq(p0, *p2) >= distSq(p0, *p1))? -1 : 1;

   return (o == 2)? -1: 1;
}

void convexHull(vector<Point> &points, int n)
{
   int ymin = points[0].y, min = 0;
   for (int i = 1; i < n; i++)
   {
     int y = points[i].y;

     if ((y < ymin) || (ymin == y &&
         points[i].x < points[min].x))
        ymin = points[i].y, min = i;
   }

   swap(points[0], points[min]);

   p0 = points[0];
   qsort(&points[1], n-1, sizeof(Point), compare);
   int m = 1;
   for (int i=1; i<n; i++)
   {
       while (i < n-1 && orientation(p0, points[i],
                                    points[i+1]) == 0)
          i++;


       points[m] = points[i];
       m++;
   }

   if (m < 3) return;

   stack<Point> st;
   st.push(points[0]);
   st.push(points[1]);
   st.push(points[2]);

   for (int i = 3; i < m; i++)
   {
      while (st.size()>1 && orientation(nextToTop(st),st.top(), points[i]) != 2)
         st.pop();
      st.push(points[i]);
   }
   stack<Point> st2;
   while (!st.empty())
   {
       st2.push(st.top());
       st.pop();
   }
   fout<<st2.size()<<'\n';
   while(!st2.empty()){
     fout<<st2.top().x<<' '<<st2.top().y<<'\n';
     st2.pop();
   }
}

int main()
{
    int n;
    fin>>n;
    std::vector<Point> v;
    int a,b;
    for(int i=1;i<=n;i++){
      fin>>a>>b;
      v.push_back({a,b});
    }
    convexHull(v, n);
    return 0;
}