Language: C++

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

int main()
{
    int N;
    long long K;
    cin>>N>>K;
    vector<long long> A(N);
    for (long long &a: A)
        cin>>a;

    vector<long long> pos;
    vector<long long> neg;
    long long zero = 0;
    for (long long a: A)
        if (a<0)
            neg.push_back(-a);
        else if (a>0)
            pos.push_back(a);
        else
            zero++;
    sort(pos.begin(), pos.end());
    sort(neg.begin(), neg.end());

    //  return #AiAj<=x
    auto count = [&](long long x)
    {
        long long c = 0;
        if (x<0)
        {
            for (long long p: pos)
                c += neg.end() - lower_bound(neg.begin(), neg.end(), (-x+p-1)/p);
        }
        else if (x==0)
        {
            c += neg.size()*pos.size();
            c += zero*(pos.size()+neg.size()) + zero*(zero-1)/2;
        }
        else
        {
            c += neg.size()*pos.size();
            c += zero*(pos.size()+neg.size()) + zero*(zero-1)/2;
            for (int i=0; i<(int)pos.size(); i++)
                c += max(0LL, lower_bound(pos.begin(), pos.end(), x/pos[i]+1)-pos.begin()-i-1);
            for (int i=0; i<(int)neg.size(); i++)
                c += max(0LL, lower_bound(neg.begin(), neg.end(), x/neg[i]+1)-neg.begin()-i-1);
        }
        return c;
    };

    long long ans = -0x2000'0000'0000'0000LL;
    for (long long b = 0x4000'0000'0000'0000LL; b>0; b/=2)
        if (count(ans+b)<K)
            ans += b;
    cout<<ans+1<<endl;
}


Compiler output:
a.cc: In lambda function:
a.cc:47:25: 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)'
   47 |                 c += max(0LL, lower_bound(pos.begin(), pos.end(), x/pos[i]+1)-pos.begin()-i-1);
      |                      ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:51,
                 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_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:47:25: 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'})
   47 |                 c += max(0LL, lower_bound(pos.begin(), pos.end(), x/pos[i]+1)-pos.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,
                 from a.cc:3:
/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:47:25: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   47 |                 c += max(0LL, lower_bound(pos.begin(), pos.end(), x/pos[i]+1)-pos.begin()-i-1);
      |                      ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:49:25: 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)'
   49 |                 c += max(0LL, lower_bound(neg.begin(), neg.end(), x/neg[i]+1)-neg.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:49:25: 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'})
   49 |                 c += max(0LL, lower_bound(neg.begin(), neg.end(), x/neg[i]+1)-neg.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:49:25: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   49 |                 c += max(0LL, lower_bound(neg.begin(), neg.end(), x/neg[i]+1)-neg.begin()-i-1);
      |                      ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~