Language: C++

#include <iostream>
#include <complex>
#include <vector>
#include <iomanip>
using namespace std;

//???????????¨
bool operator<(complex<double> a,complex<double> b){
  return a.real()<b.real() || (a.real()==b.real() && a.imag()<b.imag());
}

double cp(complex<double> a, complex<double> b){
  return (conj(a)*b).imag();
}

int main(){
  int n;
  cin >> n;

  vector<complex<double> > v; 
  for(int i=0;i<n;i++){
    int x, y;
    cin >> x >> y;
    v.push_back(complex<double>(x+0.0,y+0.0));
  }

  sort(v.begin(),v.end());

  vector<complex<double> > u,b;
  u.push_back(v[0]);
  int i;
  for(i=0;i<n;i++){
    if(v[i].real()!=v[0].real()) break;
  }
  b.push_back(v[0]);
  b.push_back(v[i]);

  i=i+1;
  while(b.back()!=v.back()){
    b.push_back(v[i]);
    i++;

    int l=b.size();
    if(l<3) continue;
    if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
    else{
      b.pop_back();
      b.pop_back();
      i--;
    }
  }
  
  i=1;
  while(u.back()!=v.back()){
    u.push_back(v[i]);
    i++;

    int l=u.size();
    if(l<3) continue;
    if(cp(u[l-1]-u[l-2],u[l-3]-u[l-2]<=0)) continue;
    else{
      u.pop_back();
      u.pop_back();
      i--;
    }
  }

  double min=b[0].imag(), index=0;
  for(int i=1;i<b.size();i++){
    if(b[i].imag()<min){
      min=b[i].imag();
      index=i;
    }
  }

  cout << (b.size()+u.size()-2) << endl;
  for(int i=index;i<b.size();i++)
    cout << noshowpoint << b[i].real() << ' ' << b[i].imag() << endl;
  for(int i=u.size()-2;i>=1;i--)
    cout << noshowpoint << u[i].real() << ' ' << u[i].imag() << endl;
  for(int i=0;i<index;i++)
    cout << noshowpoint << b[i].real() << ' ' << b[i].imag() << endl;

  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:27:3: error: 'sort' was not declared in this scope; did you mean 'sqrt'?
   27 |   sort(v.begin(),v.end());
      |   ^~~~
      |   sqrt
a.cc:45:38: error: no match for 'operator>=' (operand types are 'std::complex<double>' and 'int')
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                      ^~~
      |                                        |
      |                                        int
In file included from /usr/include/c++/14/string:48,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:476:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator>=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  476 |     operator>=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:476:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::reverse_iterator<_Iterator>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/stl_iterator.h:520:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator>=(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  520 |     operator>=(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:520:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::reverse_iterator<_Iterator>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/stl_iterator.h:1724:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator>=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1724 |     operator>=(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1724:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::move_iterator<_IteratorL>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/stl_iterator.h:1781:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator>=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1781 |     operator>=(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1781:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::move_iterator<_IteratorL>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1070:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator>=(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1070 |     operator>=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1070:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::pair<_T1, _T2>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:739:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator>=(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  739 |     operator>=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:739:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/string_view:746:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator>=(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  746 |     operator>=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:746:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/string_view:754:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator>=(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  754 |     operator>=(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:754:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/basic_string.h:3997:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator>=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3997 |     operator>=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3997:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/basic_string.h:4011:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator>=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 4011 |     operator>=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:4011:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/basic_string.h:4024:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator>=(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 4024 |     operator>=(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:4024:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   mismatched types 'const _CharT*' and 'std::complex<double>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
In file included from /usr/include/c++/14/bits/memory_resource.h:47,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/tuple:2631:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator>=(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2631 |     operator>=(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2631:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::tuple<_UTypes ...>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:3:
/usr/include/c++/14/bits/stl_vector.h:2114:5: note: candidate: 'template<class _Tp, class _Alloc> bool std::operator>=(const vector<_Tp, _Alloc>&, const vector<_Tp, _Alloc>&)'
 2114 |     operator>=(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:2114:5: note:   template argument deduction/substitution failed:
a.cc:45:40: note:   'std::complex<double>' is not derived from 'const std::vector<_Tp, _Alloc>'
   45 |     if(cp(b[l-1]-b[l-2],b[l-3]-b[l-2]>=0)) continue;
      |                                        ^
a.cc:60:38: error: no match for 'operator<=' (operand types are 'std::complex<double>' and 'int')
   60 |     if(cp(u[l-1]-u[l-2],u[l-3]-u[l-2]<=0)) continue;
      |                                      ^~~
      |                                        |
      |                                        int
/usr/include/c++/14/bits/stl_iterator.h:469:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  469 |     operator<=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:469:5: note:   template argument deduction/substitution failed:
a.cc:60:40: note:   'std::complex<double>' is not derived from 'const std::reverse_iterator<_Iterator>'
   60 |     if(cp(u[l-1]-u[l-2],u[l-3]-u[l-2]<=0)) continue;
      |                                        ^
/usr/include/c++/14/bits/stl_iterator.h:513:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<=(const