Language: C++

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

const int COUNTER_CLOCKWISE = +1;
const int CLOCKWISE = -1;
const int ONLINE_BACK = +2;
const int ONLINE_FRONT = -2;
const int ON_SEGMENT = 0;

#define EPS 1e-10
#define INF 1e10

typedef complex<double> Point;
typedef complex<double> Vector;
typedef vector<Point> Polygon;

struct Line
{
	Point a, b;
	Line() {}
	Line(const Point &_a, const Point &_b)
	{
		a = _a; b = _b;
	}
};

//Line????????????
struct Segment
{
	Point a, b;
	Segment() {}
	Segment(const Point &_a, const Point &_b)
	{
		a = _a; b = _b;
	}
};

struct Circle
{
	Point c;
	double r;

	Circle() {}

	Circle(const Point &_c, const double &_r)
	{
		c = _c; r = _r;
	}
};

istream& operator >> (istream& is, Point &p)
{
	double x, y;
	is >> x >> y;
	p.real(x); p.imag(y);
	return is;
}

Segment ToSegment(const Line l)
{
	return Segment(l.a, l.b);
}

Line ToLine(const Segment s)
{
	return Line(s.a, s.b);
}

//???????¨±???==
bool equals(const double &a, const double &b)
{
	return abs(a - b) < EPS;
}

//???????¨±???==
bool equals(const Vector &a, const Vector &b)
{
	return equals(a.real(), b.real()) && equals(a.imag(), b.imag());
}

//Point?????????
bool comp(const Point &a, const Point &b)
{
	//return equals(a.real(), b.real()) ? a.imag() < b.imag() : a.real() < b.real();
	return equals(a.imag(), b.imag()) ? a.real() < b.real() : a.imag() < b.imag();
}

//??????
double dot(const Vector &a, const Vector &b)
{
	return a.real()*b.real() + a.imag()*b.imag();
}

//??????
double cross(const Vector &a, const Vector &b)
{
	return a.real()*b.imag() - a.imag()*b.real();
}

//????§???¢?????¢???
double GetArea(const Polygon &g)
{
	double res = 0;
	for (int i = 1; i < g.size() - 1; i++)
	{
		res += cross(g[i] - g[0], g[i + 1] - g[0]);
	}
	return abs(res / 2);
}

//???????¨???????(O(1))
//?????¶:????????¨?????°??????
int CCW(const Point &p0, const Point &p1, const Point &p2)
{
	Vector v1 = (Vector)(p1 - p0), v2 = p2 - p0;
	if (cross(v1, v2) > EPS) return COUNTER_CLOCKWISE;
	if (cross(v1, v2) < -EPS) return CLOCKWISE;
	if (dot(v1, v2) < -EPS) return ONLINE_BACK;
	if (abs(v1) < abs(v2)) return ONLINE_FRONT;
	return ON_SEGMENT;
}

//??´?????????(O(1))
bool IsOrthogonal(const Vector &a, const Vector &b)
{
	return equals(dot(a, b), 0.0);
}
bool IsOrthogonal(const Line &l1, const Line &l2)
{
	return equals(dot(l1.b - l1.a, l2.b - l2.a), 0.0);
}

//????????????(O(1))
bool IsParallel(const Vector &a, const Vector &b)
{
	return equals(cross(a, b), 0.0);
}
bool IsParallel(const Line &l1, const Line &l2)
{
	return equals(cross(l1.b - l1.a, l2.b - l2.a), 0.0);
}
//
////?°???±(O(1))
//Point Project(const Line &s, const Point &p) {
//	Vector v1 = s.b - s.a;
//	Vector v2 = p - s.a;
//	double r = dot(v1, v2) / norm(v1);
//	return s.a + v1 * r;
//}
//
////????°?(O(1))
//Point Reflect(const Line &s, const Point &p) {
//	return p + (Project(s, p) - p) * 2.0;
//}

//??´?????¨?????????????????????(O(1))
bool Intersects(const Line &l, const Segment &s)
{
	return CCW(l.a, l.b, s.a)*CCW(l.a, l.b, s.b) != 1;
}

//??????????????????
bool Intersects(const Segment &l1, const Segment &l2)
{
	return CCW(l1.a, l1.b, l2.a)*CCW(l1.a, l1.b, l2.b) <= 0 &&
		CCW(l2.a, l2.b, l1.a)*CCW(l2.a, l2.b, l1.b) <= 0;
}

//??´?????¨??´????????????(O(1))
Point GetIntersection(const Line &l1, const Line &l2)
{
	Vector a1 = l1.b - l1.a, a2 = l2.b - l2.a;
	Vector b1 = l2.a - l1.a, b2 = l1.a - l2.b;
	double s1 = cross(a1, b1) / 2, s2 = cross(a1, b2) / 2;
	return Point(l2.a.real() + a2.real()*s1 / (s1 + s2), l2.a.imag() + a2.imag()*s1 / (s1 + s2));
}

//?????¨????????????
//?????????
//	???????????????????????°
vector<Point> GetIntersection(const Circle &c1, const Circle &c2)
{
	vector<Point> res;
	double d, a, t;
	d = sqrt(norm((c2.c - c1.c)));
	if (d < abs(c1.r - c2.r) || c1.r + c2.r < d) return res; //?????????0
	a = acos((c1.r*c1.r + d*d - c2.r*c2.r) / (2 * c1.r*d));
	t = atan2(c2.c.imag() - c1.c.imag(), c2.c.real() - c1.c.real());
	res.push_back(Point(c1.c.real() + c1.r*cos(t + a), c1.c.imag() + c1.r*sin(t + a)));
	res.push_back(Point(c1.c.real() + c1.r*cos(t - a), c1.c.imag() + c1.r*sin(t - a)));
	if (equals(res[0], res[1])) res.pop_back(); //?????????1
	return res;
}

//????§???¢????????????(O(n))
//?????????
//  0:???????????????
//  1:????????????
//  2:?????????
int Contains(const Polygon &g, const Point &p)
{
	Line l = Line(p, Point(INF, p.imag()));
	int cnt = 0, n = g.size();
	for (int i = 0; i < n; i++)
	{
		Vector a = g[i] - p;
		Vector b = g[(i + 1) % n] - p;
		if (CCW(g[i], g[(i + 1) % n], p) == 0) return 2; //?????????
		if (a.imag() > b.imag()) swap(a, b);
		if (a.imag() <= EPS && EPS < b.imag() && cross(a, b) > EPS) cnt++;
	}
	if ((cnt & 1) == 1) return 1; //????????????
	return 0; //???????????????
}

//??????(O(n))
//?????????
//	x->y????????§???????°??????????????????????????¨???????
Polygon AndrewScan(Polygon &s)
{
	if (s.size() <= 2) return s;
	sort(s.begin(), s.end(), comp);
	Polygon g;
	for (int i = 0; i < s.size(); i++)
	{
		//CCW?????¨?????????==COUNTER_CLOCKWISE???????????¨???????????????
		for (int n = g.size(); n >= 2 && CCW(g[n - 2], g[n - 1], s[i]) == COUNTER_CLOCKWISE; n--)
		{
			g.pop_back();
		}
		g.push_back(s[i]);
	}
	for (int i = s.size() - 2; i >= 0; i--)
	{
		for (int n = g.size(); CCW(g[n - 2], g[n - 1], s[i]) == COUNTER_CLOCKWISE; n--)
		{
			g.pop_back();
		}
		g.push_back(s[i]);
	}
	reverse(g.begin(), g.end());
	g.pop_back();
	return g;
}

//???????§???¢?????????(O(n))
Polygon ConvexCut(const Polygon &g, const Line &l)
{
	Polygon res;
	for (int i = 0; i < g.size(); i++)
	{
		Segment s = Segment(g[i], g[(i + 1) % g.size()]);
		if (CCW(l.a, l.b, s.a) != -1) res.push_back(s.a);
		if (CCW(l.a, l.b, s.a)*CCW(l.a, l.b, s.b) == -1) 
			res.push_back(GetIntersection(l, ToLine(s)));
	}
	return res;
}

int main(){
    int N;
    Polygon S,g;
    cin>>N; S.resize(N);
    for(int i=0;i<N;i++) S[i].input();
    g = andrewScan(S);
    int n = g.size();
    printf("%d\n",n);
    int sn; Point s = Point(INF,INF);
    for(int i=0;i<n;i++){
        if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
            sn = i; s = g[i];
        }
    }
    for(int i=0;i<n;i++){
        printf("%d %d\n",(int)g[(i+sn)%n].x,(int)g[(i+sn)%n].y);
    }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:265:31: error: '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'class std::complex<double>'} has no member named 'input'
  265 |     for(int i=0;i<N;i++) S[i].input();
      |                               ^~~~~
a.cc:266:9: error: 'andrewScan' was not declared in this scope; did you mean 'AndrewScan'?
  266 |     g = andrewScan(S);
      |         ^~~~~~~~~~
      |         AndrewScan
a.cc:271:17: error: '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'class std::complex<double>'} has no member named 'y'
  271 |         if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
      |                 ^
a.cc:271:21: error: 'Point' {aka 'class std::complex<double>'} has no member named 'y'
  271 |         if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
      |                     ^
a.cc:271:32: error: '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'class std::complex<double>'} has no member named 'y'
  271 |         if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
      |                                ^
a.cc:271:37: error: 'Point' {aka 'class std::complex<double>'} has no member named 'y'
  271 |         if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
      |                                     ^
a.cc:271:47: error: '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'class std::complex<double>'} has no member named 'x'
  271 |         if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
      |                                               ^
a.cc:271:51: error: 'Point' {aka 'class std::complex<double>'} has no member named 'x'
  271 |         if(g[i].y<s.y || (g[i].y==s.y && g[i].x<s.x)){
      |                                                   ^
a.cc:276:43: error: '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'class std::complex<double>'} has no member named 'x'
  276 |         printf("%d %d\n",(int)g[(i+sn)%n].x,(int)g[(i+sn)%n].y);
      |                                           ^
a.cc:276:62: error: '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'class std::complex<double>'} has no member named 'y'
  276 |         printf("%d %d\n",(int)g[(i+sn)%n].x,(int)g[(i+sn)%n].y);
      |                                                              ^