Convex Hull

#include <bits/stdc++.h>
#define int long long

using namespace std;

class point {
    public:
        int x, y;

        void read() {
            cin >> x >> y;
        }

        point operator - (const point &p) const {
            return point{x - p.x, y - p.y};
        }

        void operator -= (const point &p) {
            x -= p.x, y -= p.y;
        }

        int operator * (const point &p) const {
            return x * p.y - p.x * y;
        }

        int crossP(const point &A, const point &B) const {
            return (A - *this) * (B - *this);
        }

        bool operator < (const point &A) const {
            return make_pair(x, y) < make_pair(A.x, A.y);
        }
};

void test_case() {
    int N;
    cin >> N;
    vector<point> points(N);
    for(point &p : points)
        p.read();
    sort(points.begin(), points.end());
    vector<point> hull;
    for(int rep = 0; rep < 2; ++rep) {
        int S = hull.size();
        for(const point &C : points) {
            while((int)hull.size() > S + 1) {
                point A = hull.end()[-2],
                      B = hull.end()[-1];
                if(A.crossP(B, C) <= 0) // B la stanga lui C sau A - B - C - coliniare
                    break;
                hull.pop_back(); // daca e la dreapta lui C nu face parte din convex hull
            }
            hull.emplace_back(C);
        }
        hull.pop_back();
        reverse(points.begin(), points.end());
    }
    cout << hull.size() << '\n';
    for(const point &p : hull)
        cout << p.x << ' ' << p.y << '\n';
}

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int T = 1;
    while(T--)
        test_case();
}