Language: C++

#include <bits/stdc++.h>
#define ALL(v) v.begin(), v.end()
#define MOD 1000000007
#define INF 1000000007
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define req(i, j, n) for(int i = j; i <= (int)(n); i++)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;

int pw2(int n){
  int t=1;
  while(t>n){
    t*=2;}
  return t;
}

int main() {
  int n;
  cin>>n;
  
  map<int, int> nu;
  
  for(int i=1;i<=n;i++){
   int a;
    cin>>a;
    if(nu.find(a)==nu.end())
      nu[a]=0;
    ++nu[a];
  }
  
  int ans=0;
  for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
    int b=itr->first;
    int p=pw2(b)-b;
    
    if(b==p){
      ans+=nu[b]/2;}
    else{
      if(nu,find(p) == nu.end())continue;
      ans+=min(nu[b], nu[p]);
      nu[p]-=min(nu[b], nu[p]);
    }
  }
  
  cout<<ans<<endl;
  
  
  
  
  
  


}


Compiler output:
a.cc: In function 'int main()':
a.cc:33:42: error: no match for 'operator+' (operand types are 'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} and 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >')
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                 ~~~~~~~~~^~~~~~
      |                                        |     |
      |                                        |     reverse_iterator<[...]>
      |                                        reverse_iterator<[...]>
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 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_iterator.h:340:7: note: candidate: 'constexpr std::reverse_iterator<_Iterator> std::reverse_iterator<_Iterator>::operator+(difference_type) const [with _Iterator = std::_Rb_tree_iterator<std::pair<const int, int> >; difference_type = long int]'
  340 |       operator+(difference_type __n) const
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:340:33: note:   no known conversion for argument 1 from 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >' to 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >::difference_type' {aka 'long int'}
  340 |       operator+(difference_type __n) const
      |                 ~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_iterator.h:627:5: note: candidate: 'constexpr std::reverse_iterator<_Iterator> std::operator+(typename reverse_iterator<_Iterator>::difference_type, const reverse_iterator<_Iterator>&) [with _Iterator = _Rb_tree_iterator<pair<const int, int> >; typename reverse_iterator<_Iterator>::difference_type = long int]'
  627 |     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:627:69: note:   no known conversion for argument 1 from 'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} to 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >::difference_type' {aka 'long int'}
  627 |     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note: candidate: 'template<class _Iterator> constexpr std::move_iterator<_IteratorL> std::operator+(typename move_iterator<_IteratorL>::difference_type, const move_iterator<_IteratorL>&)'
 1798 |     operator+(typename move_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >' is not derived from 'const std::move_iterator<_IteratorL>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/bits/basic_string.h:3598:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3598 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3598:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3616:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3616 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3616:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   mismatched types 'const _CharT*' and 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3635:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3635 |     operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3635:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3652:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3652 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3652:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3670:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, _CharT)'
 3670 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, _CharT __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3670:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3682:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3682 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3682:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3689:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3689 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3689:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3696:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3696 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3696:5: note:   template argument deduction/substitution failed:
a.cc:33:46: note:   'std::map<int, int>::reverse_iterator' {aka 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >'} is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   33 |   for(auto itr=nu.rbegin();itr!=nu.rend()+itr++){
      |                                              ^~
/usr/include/c++/14/bits/basic_string.h:3719:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3719 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3719:5: note:   template argument deduction/substitution failed: