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].imag()<s.imag() || (g[i].imag()==s.imag() && g[i].real()<s.real())){
            sn = i; s = g[i];
        }
    }
    for(int i=0;i<n;i++){
        printf("%d %d\n",(int)g[(i+sn)%n].real(),(int)g[(i+sn)%n].imag());
    }
}

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();
      |                               ^~~~~