Language: C++

#include <iostream>
#include <set>

using namespace std;

multiset < int > s;

int bs(int val) {
    int pas = 16, r(-1);
    while (pas) {
        if (r + pas < 30 && (1<<(r + pas)) <= val)
            r += pas;
        pas >>= 1;
    }
    return (1<<(r + 1));
}

int main()
{
    int n, val, ans(0);
    cin >> n;
    for (int i = 0; i < n; ++i)
        cin >> val, s.insert(val);
    while (s.size()) {
        int x = bs(*s.rbegin()) - *s.rbegin();
        s.erase(s.rbegin());
        if (s.find(x) != s.end())
            s.erase(x), ++ans;
    }
    cout << ans;
    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:26:16: error: no matching function for call to 'std::multiset<int>::erase(std::multiset<int>::reverse_iterator)'
   26 |         s.erase(s.rbegin());
      |         ~~~~~~~^~~~~~~~~~~~
In file included from /usr/include/c++/14/set:64,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_multiset.h:641:7: note: candidate: 'std::multiset<_Key, _Compare, _Alloc>::iterator std::multiset<_Key, _Compare, _Alloc>::erase(const_iterator) [with _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>; iterator = std::_Rb_tree<int, int, std::_Identity<int>, std::less<int>, std::allocator<int> >::const_iterator; const_iterator = std::_Rb_tree<int, int, std::_Identity<int>, std::less<int>, std::allocator<int> >::const_iterator]'
  641 |       erase(const_iterator __position)
      |       ^~~~~
/usr/include/c++/14/bits/stl_multiset.h:641:28: note:   no known conversion for argument 1 from 'std::multiset<int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_const_iterator<int> >'} to 'std::multiset<int>::const_iterator' {aka 'std::_Rb_tree<int, int, std::_Identity<int>, std::less<int>, std::allocator<int> >::const_iterator'}
  641 |       erase(const_iterator __position)
      |             ~~~~~~~~~~~~~~~^~~~~~~~~~
/usr/include/c++/14/bits/stl_multiset.h:671:7: note: candidate: 'std::multiset<_Key, _Compare, _Alloc>::size_type std::multiset<_Key, _Compare, _Alloc>::erase(const key_type&) [with _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>; size_type = long unsigned int; key_type = int]'
  671 |       erase(const key_type& __x)
      |       ^~~~~
/usr/include/c++/14/bits/stl_multiset.h:671:29: note:   no known conversion for argument 1 from 'std::multiset<int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_const_iterator<int> >'} to 'const std::multiset<int>::key_type&' {aka 'const int&'}
  671 |       erase(const key_type& __x)
      |             ~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_multiset.h:693:7: note: candidate: 'std::multiset<_Key, _Compare, _Alloc>::iterator std::multiset<_Key, _Compare, _Alloc>::erase(const_iterator, const_iterator) [with _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>; iterator = std::_Rb_tree<int, int, std::_Identity<int>, std::less<int>, std::allocator<int> >::const_iterator; const_iterator = std::_Rb_tree<int, int, std::_Identity<int>, std::less<int>, std::allocator<int> >::const_iterator]'
  693 |       erase(const_iterator __first, const_iterator __last)
      |       ^~~~~
/usr/include/c++/14/bits/stl_multiset.h:693:7: note:   candidate expects 2 arguments, 1 provided