Cod sursă (job #807222)

#include <fstream>
#include <algorithm>

using namespace std;

ifstream f("infasuratoare.in");
ofstream g("infasuratoare.out");

struct Punct
{
    int x, y;
};

Punct P[100002];///St[1002];
int N, H, St[100002];

void afis()
{
    g << H << '\n';
    for(int i = 1; i <= H; i++)
        g << P[St[i]].x << ' ' << P[St[i]].y << '\n';
}

inline long long patrat(long long x)
{
    return x * x;
}

long long distp(const Punct &A, const Punct &B)
{
    return patrat(A.x - B.x) + patrat(A.y - B.y);
}

bool compy(const Punct &A, const Punct &B)
{
    if(A.y == B.y) return A.x < B.x;
    return A.y < B.y;
}

long long det(const Punct &A, const Punct &B, const Punct &C)
{
    return (long long)(A.x - B.x) * (B.y - C.y) - (long long)(A.y - B.y) * (B.x - C.x);
}

bool compd(const Punct &A, const Punct &B)
{
    long long dd = det(P[1], A, B);
    if(dd == 0)
        return distp(P[1], A) < distp(P[1], B);
    return dd > 0;
}

int main()
{
    f >> N;
    int pmin = 1;
    for(int i = 1; i <= N; i++)
    {
        f >> P[i].x >> P[i].y;
        if(compy(P[i], P[pmin]))
            pmin = i;
    }
    swap(P[pmin], P[1]);
    sort(P + 2, P + N + 1, compd);
    St[1] = 1;
    St[2] = 2;
    H = 2;
    P[N + 1] = P[1];
    for(int i = 3; i <= N + 1; i++)
    {
        while(H >= 2 && det(P[St[H - 1]], P[St[H]], P[i]) <= 0)
            H--;
        St[++H] = i;
    }
    H--;
    afis();
    return 0;
}