ConvexHull

#include <cmath>
#include <iostream>
#include <vector>

using namespace std;

struct Point {
  double x;
  double y;
};

struct Vector {
  double x;
  double y;

  double Len() const;
};

Point operator+(const Point& p, const Vector& v) {
  return {p.x + v.x, p.y + v.y};
}

Vector operator+(const Vector& v1, const Vector& v2) {
  return {v1.x + v2.x, v1.y + v2.y};
}

Vector operator-(const Vector& v1, const Vector& v2) {
  return {v1.x - v2.x, v1.y - v2.y};
}

Vector operator-(const Point& p1, const Point& p2) {
  return {p1.x - p2.x, p1.y - p2.y};
}

double DotProduct(const Vector& v1, const Vector& v2) {
  return v1.x * v2.x + v1.y * v2.y;
}

double CrossProduct(const Vector& v1, const Vector& v2) {
  return v1.x * v2.y - v1.y * v2.x;
}

double Vector::Len() const {
  return sqrt(x * x + y * y);
}

bool operator==(const Point& p1, const Point& p2) {
  return p1.x == p2.x && p1.y == p2.y;
}

bool operator!=(const Point& p1, const Point& p2) {
  return !operator==(p1, p2);
}

istream& operator>>(istream& in, Vector& v) {
  in >> v.x >> v.y;
  return in;
}

istream& operator>>(istream& in, Point& v) {
  in >> v.x >> v.y;
  return in;
}

vector<Point> ConvexHull(vector<Point>& p) {
  if (p.empty()) return {};
  auto it = min_element(p.begin(), p.end(), [](const Point& p1, const Point& p2) -> bool {
    return p1.y < p2.y || (p1.y == p2.y && p1.x < p2.x);
  });

  vector<Point> ch;
  ch.push_back(*it);

  for (;;) {
    Point cur = ch.back();
    Point cand = p.front();
    for (int i = 1; i < p.size(); ++i) {
      double cross = CrossProduct(cand - cur, p[i] - cur);
      if (cross < -eps || (cross < eps && (p[i] - cur).Len() > (cand - cur).Len())) cand = p[i];
    }
    if (cand == ch.front()) return ch;
    ch.push_back(cand);
  }
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n = 0; cin >> n;
  vector<Point> p(n);
  for (int i = 0; i < n; ++i) cin >> p[i];
  vector<Point> ch = ConvexHull(p);
  for (const auto& point : ch) {
    cout << point.x << " " << point.y << "\n";
  }
  return 0;
}