Language: C++

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;
typedef long long ll;
const ll ll_mod=1e9+7;
int n,k;

bool compare_by_b(pair<int, int> a, pair<int, int> b) {
    if(a.second != b.second){
        return a.second > b.second;
    }else{
        return a.first < b.first;
    }
}

int main(){
    cin >> n >> k;
    vector< pair<ll,ll> > a(n);
    bool all_negative=true;
    for(int i=0; i<n; i++){
        cin >> a[i].first;
        a[i].second = abs(a[i].first);
        if(a[i].first>0) all_negative=false;
    }
    ll ans=1;
    //k=nのとき
    if(k==n){
        for(int i=0; i<n; i++){ans *= a[i].first; ans %= ll_mod;}
        if(ans<0){ans += ll_mod; ans%=ll_mod;}
        cout << ans << "\n";
        return 0;
    }
    //sort
    sort(a.begin(), a.end(), compare_by_b);
    //全てのaが負でかつkが奇数のとき
    if(all_negative && k%2==1){
        for(int i=n-1; i>=n-k; i--){ans *= a[i].first; ans %= ll_mod;}
        if(ans<0) ans += ll_mod;
        else if(a[n-1]==0) ans=0;
        cout << ans << "\n";
        return 0;
    }
    //それ以外
    for(int i=0; i<k; i++){
        ans *= a[i].first; ans %= ll_mod;
        if(a[i].first>0) all_negative=false;
    }
    if(ans<0){
        ll rm_negative, rm_positive, add_negative, add_positive;
        rm_negative = -1e10;
        rm_positive = 1e10;
        add_negative = add_positive = 0;
        for(int i=0; i<k; i++){
            if(a[i].first<0) rm_negative = max(rm_negative, a[i].first);
            else if(a[i].first>0) rm_positive = min(rm_positive, a[i].first);
        }
        for(int i=k; i<n; i++){
            if(a[i].first<0) add_negative = min(add_negative, a[i].first);
            else if(a[i].first>0) add_positive = max(add_positive, a[i].first);
        }
        ans=1;
        if(rm_negative*add_negative<=rm_positive*add_positive || all_negative){
            for(int i=0; i<k; i++){
                if(a[i].first==rm_negative) ans*= add_positive;
                else ans *= a[i].first; 
                ans %= ll_mod;
            }
        }else{
            for(int i=0; i<k; i++){
                if(a[i].first==rm_positive) ans*= add_negative;
                else ans *= a[i].first; 
                ans %= ll_mod;
            }
        }
    }
    cout << ans << "\n";
}


Compiler output:
a.cc: In function 'int main()':
a.cc:41:23: error: no match for 'operator==' (operand types are '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} and 'int')
   41 |         else if(a[n-1]==0) ans=0;
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:192:5: note: candidate: 'template<class _StateT> bool std::operator==(const fpos<_StateT>&, const fpos<_StateT>&)'
  192 |     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:192:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::fpos<_StateT>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
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:235:5: note: candidate: 'template<class _T1, class _T2> bool std::operator==(const allocator<_CharT>&, const allocator<_T2>&)'
  235 |     operator==(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:235:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::allocator<_CharT>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1667 |     operator==(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::move_iterator<_IteratorL>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1737 |     operator==(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::move_iterator<_IteratorL>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
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:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
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:629:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  629 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:629:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   'std::pair<long long int, long long int>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/string_view:637:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  637 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:637:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   'std::pair<long long int, long long int>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/string_view:644:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  644 |     operator==(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:644:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/bits/basic_string.h:3755:5: 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>&)'
 3755 |     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3755:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/bits/basic_string.h:3772:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3772 |     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3772:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::pair<long long int, long long int> >, std::pair<long long int, long long int> >::value_type' {aka 'std::pair<long long int, long long int>'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
/usr/include/c++/14/bits/basic_string.h:3819:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3819 |     operator==(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3819:5: note:   template argument deduction/substitution failed:
a.cc:41:25: note:   mismatched types 'const _CharT*' and 'std::pair<long long int, long long int>'
   41 |         else if(a[n-1]==0) ans=0;
      |                         ^
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:2558:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator==(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2558 |     operator==(const tuple<_TElements...>& __t,