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[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:27:13: error: no match for 'operator==' (operand types are 'std::map<int, int>::mapped_type' {aka 'int'} and 'std::map<int, int>::iterator' {aka 'std::_Rb_tree<int, std::pair<const int, int>, std::_Select1st<std::pair<const int, int> >, std::less<int>, std::allocator<std::pair<const int, int> > >::iterator'})
   27 |     if(nu[a]==nu.end())
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1103:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1103 |     operator==(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1103:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:1199:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator==(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1199 |     operator==(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1199:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   mismatched types 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:1274:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator==(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1274 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1274:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:1366:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1366 |     operator==(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1366:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   'std::map<int, int>::iterator' {aka 'std::_Rb_tree<int, std::pair<const int, int>, std::_Select1st<std::pair<const int, int> >, std::less<int>, std::allocator<std::pair<const int, int> > >::iterator'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:1441:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1441 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1441:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:1534:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1534 |     operator==(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1534:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   'std::map<int, int>::iterator' {aka 'std::_Rb_tree<int, std::pair<const int, int>, std::_Select1st<std::pair<const int, int> >, std::less<int>, std::allocator<std::pair<const int, int> > >::iterator'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:1613:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1613 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1613:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/usr/include/c++/14/bits/regex.h:2186:5: note: candidate: 'template<class _Bi_iter, class _Alloc> bool std::__cxx11::operator==(const match_results<_BiIter, _Alloc>&, const match_results<_BiIter, _Alloc>&)'
 2186 |     operator==(const match_results<_Bi_iter, _Alloc>& __m1,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:2186:5: note:   template argument deduction/substitution failed:
a.cc:27:22: note:   mismatched types 'const std::__cxx11::match_results<_BiIter, _Alloc>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h: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:27:22: note:   mismatched types 'const std::pair<_T1, _T2>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/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:27:22: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/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:27:22: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/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:27:22: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
/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:27:22: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
In file included from /usr/include/c++/14/bits/char_traits.h:42,
                 from /usr/include/c++/14/string:42,
                 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/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:27:22: note:   mismatched types 'const std::fpos<_StateT>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
In file included from /usr/include/c++/14/string:43:
/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:27:22: note:   mismatched types 'const std::allocator<_CharT>' and 'std::map<int, int>::mapped_type' {aka 'int'}
   27 |     if(nu[a]==nu.end())
      |                      ^
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: