Language: C++

#include<bits/stdc++.h>
using namespace std;
int main(){
    const long long inf=2000000000000000000;
    int n;
    long long k;
    cin>>n>>k;
    int a[n];
    vector<long long>sei;
    vector<long long>fu;
    for(int i=0;i<n;i++){
        cin>>a[i];
        if(a[i]>0)sei.push_back(a[i]);
        if(a[i]<0)fu.push_back(-a[i]);
    }
    sort(sei.begin(),sei.end());
    sort(fu.begin(),fu.end());
    long long minus,zero,plus;
    minus=sei.size()*fu.size();
    plus=(sei.size()*(sei.size()-1)+fu.size()*(fu.size()-1))/2;
    zero=n*(n-1)/2-minus-plus;
    if(k>minus && k<=minus+zero){
        cout<<"0"<<endl;
        return 0;
    }
    if(k<=minus){//解が負
        long long l=-inf,r=0;
        while(r-l>1){
            long long mid=-(l+r)/2,x=0;
            for(int i=0;i<fu.size();i++)x+=sei.size()-(upper_bound(sei.begin(),sei.end(),mid/fu[i])-sei.begin());
            if(x<k)l=-mid;
            else r=-mid;
        }
        cout<<l<<endl;
    }
    else{//解が正
        k-=minus+zero;
        long long l=1,r=inf;
        while(r-l>1){
            long long mid=(l+r)/2,x=0;
            for(int i=0;i<fu.size();i++)x+=max(0ll,upper_bound(fu.begin(),fu.end(),(mid-1)/fu[i])-fu.begin()-i-1);
            for(int i=0;i<sei.size();i++)x+=max(0ll,upper_bound(sei.begin(),sei.end(),(mid-1)/sei[i])-sei.begin()-i-1);
            if(x<k)l=mid;
            else r=mid;
        }
        cout<<l<<endl;
    }
    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:41:47: error: no matching function for call to 'max(long long int, __gnu_cxx::__normal_iterator<long long int*, std::vector<long long int> >::difference_type)'
   41 |             for(int i=0;i<fu.size();i++)x+=max(0ll,upper_bound(fu.begin(),fu.end(),(mid-1)/fu[i])-fu.begin()-i-1);
      |                                            ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:41:47: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and '__gnu_cxx::__normal_iterator<long long int*, std::vector<long long int> >::difference_type' {aka 'long int'})
   41 |             for(int i=0;i<fu.size();i++)x+=max(0ll,upper_bound(fu.begin(),fu.end(),(mid-1)/fu[i])-fu.begin()-i-1);
      |                                            ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:41:47: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   41 |             for(int i=0;i<fu.size();i++)x+=max(0ll,upper_bound(fu.begin(),fu.end(),(mid-1)/fu[i])-fu.begin()-i-1);
      |                                            ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:42:48: error: no matching function for call to 'max(long long int, __gnu_cxx::__normal_iterator<long long int*, std::vector<long long int> >::difference_type)'
   42 |             for(int i=0;i<sei.size();i++)x+=max(0ll,upper_bound(sei.begin(),sei.end(),(mid-1)/sei[i])-sei.begin()-i-1);
      |                                             ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:42:48: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and '__gnu_cxx::__normal_iterator<long long int*, std::vector<long long int> >::difference_type' {aka 'long int'})
   42 |             for(int i=0;i<sei.size();i++)x+=max(0ll,upper_bound(sei.begin(),sei.end(),(mid-1)/sei[i])-sei.begin()-i-1);
      |                                             ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:42:48: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   42 |             for(int i=0;i<sei.size();i++)x+=max(0ll,upper_bound(sei.begin(),sei.end(),(mid-1)/sei[i])-sei.begin()-i-1);
      |                                             ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~