Language: C++

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

//???????????¨
bool lt(const complex<double>& a,const 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(),lt);

  vector<complex<int> > 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:40:17: error: no match for 'operator!=' (operand types are '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} and '__gnu_cxx::__alloc_traits<std::allocator<std::complex<double> >, std::complex<double> >::value_type' {aka 'std::complex<double>'})
   40 |   while(b.back()!=v.back()){
      |         ~~~~~~~~^~~~~~~~~~
      |               |         |
      |               |         complex<double>
      |               complex<int>
In file included from /usr/include/c++/14/iosfwd:42,
                 from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/postypes.h:197:5: note: candidate: 'template<class _StateT> bool std::operator!=(const fpos<_StateT>&, const fpos<_StateT>&)'
  197 |     operator!=(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:197:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::fpos<_StateT>'
   40 |   while(b.back()!=v.back()){
      |                          ^
In file included from /usr/include/c++/14/string:43,
                 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:
/usr/include/c++/14/bits/allocator.h:243:5: note: candidate: 'template<class _T1, class _T2> bool std::operator!=(const allocator<_CharT>&, const allocator<_T2>&)'
  243 |     operator!=(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:243:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::allocator<_CharT>'
   40 |   while(b.back()!=v.back()){
      |                          ^
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:455:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  455 |     operator!=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:455:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/bits/stl_iterator.h:500:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  500 |     operator!=(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:500:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1686 |     operator!=(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::move_iterator<_IteratorL>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1753 |     operator!=(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::move_iterator<_IteratorL>'
   40 |   while(b.back()!=v.back()){
      |                          ^
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:1052:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator!=(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1052 |     operator!=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1052:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::pair<_T1, _T2>'
   40 |   while(b.back()!=v.back()){
      |                          ^
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:651:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  651 |     operator!=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:651:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   'std::complex<int>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/string_view:658:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  658 |     operator!=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:658:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   'std::complex<int>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/string_view:666:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  666 |     operator!=(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:666:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   'std::complex<double>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/bits/basic_string.h:3833:5: 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>&)'
 3833 |     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3833:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/bits/basic_string.h:3847:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3847 |     operator!=(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3847:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   mismatched types 'const _CharT*' and 'std::complex<int>'
   40 |   while(b.back()!=v.back()){
      |                          ^
/usr/include/c++/14/bits/basic_string.h:3860:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3860 |     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3860:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   40 |   while(b.back()!=v.back()){
      |                          ^
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:2613:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator!=(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2613 |     operator!=(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2613:5: note:   template argument deduction/substitution failed:
a.cc:40:26: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::complex<int> >, std::complex<int> >::value_type' {aka 'std::complex<int>'} is not derived from 'const std::tuple<_UTypes ...>'
   40 |