Language: C++

#include<iostream>
#include<vector>
using namespace std;
int main () {
  int N;
  cin >> N;
  vector<int>a(N);
  for (int i = 0; i < N; i ++) cin >> a[i];
  vector<int>box(N, 0);
  vector<vector<int>>re(N, vector<int>());
  vector<int>ans();
  for (int i = 1; i <= N / 2; i ++) {
    for (int j = 2; j <= N / i; j ++) {
      int k = i * j - 1;
      re[k].push_back(i - 1);
    }
  }
  for (int i = N - 1; i >= 0; i --) {
    if (box[i] % 2 != a[i]) {
      ans.push_back(i + 1);
      for (int j = 0; j < re[i].size(); j ++) {
        box[re[j]] ++;
      }
    }
  }
  cout << ans.size() << endl;
  if (!ans.size()) return 0;
  cout << ans[0] << ' '
  for (int i = 1; i < ans.size(); i ++) {
    cout << ' ' << ans[i];
  }
  cout << endl;
}
  
  

Compiler output:
a.cc: In function 'int main()':
a.cc:11:17: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
   11 |   vector<int>ans();
      |                 ^~
a.cc:11:17: note: remove parentheses to default-initialize a variable
   11 |   vector<int>ans();
      |                 ^~
      |                 --
a.cc:20:11: error: request for member 'push_back' in 'ans', which is of non-class type 'std::vector<int>()'
   20 |       ans.push_back(i + 1);
      |           ^~~~~~~~~
a.cc:22:12: error: no match for 'operator[]' (operand types are 'std::vector<int>' and '__gnu_cxx::__alloc_traits<std::allocator<std::vector<int> >, std::vector<int> >::value_type' {aka 'std::vector<int>'})
   22 |         box[re[j]] ++;
      |            ^
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_vector.h:1128:7: note: candidate: 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](size_type) [with _Tp = int; _Alloc = std::allocator<int>; reference = int&; size_type = long unsigned int]'
 1128 |       operator[](size_type __n) _GLIBCXX_NOEXCEPT
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1128:28: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<int> >, std::vector<int> >::value_type' {aka 'std::vector<int>'} to 'std::vector<int>::size_type' {aka 'long unsigned int'}
 1128 |       operator[](size_type __n) _GLIBCXX_NOEXCEPT
      |                  ~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:1147:7: note: candidate: 'std::vector<_Tp, _Alloc>::const_reference std::vector<_Tp, _Alloc>::operator[](size_type) const [with _Tp = int; _Alloc = std::allocator<int>; const_reference = const int&; size_type = long unsigned int]'
 1147 |       operator[](size_type __n) const _GLIBCXX_NOEXCEPT
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1147:28: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<int> >, std::vector<int> >::value_type' {aka 'std::vector<int>'} to 'std::vector<int>::size_type' {aka 'long unsigned int'}
 1147 |       operator[](size_type __n) const _GLIBCXX_NOEXCEPT
      |                  ~~~~~~~~~~^~~
a.cc:26:15: error: request for member 'size' in 'ans', which is of non-class type 'std::vector<int>()'
   26 |   cout << ans.size() << endl;
      |               ^~~~
a.cc:27:12: error: request for member 'size' in 'ans', which is of non-class type 'std::vector<int>()'
   27 |   if (!ans.size()) return 0;
      |            ^~~~
a.cc:28:16: warning: pointer to a function used in arithmetic [-Wpointer-arith]
   28 |   cout << ans[0] << ' '
      |                ^
a.cc:28:24: error: expected ';' before 'for'
   28 |   cout << ans[0] << ' '
      |                        ^
      |                        ;
   29 |   for (int i = 1; i < ans.size(); i ++) {
      |   ~~~                   
a.cc:29:19: error: 'i' was not declared in this scope
   29 |   for (int i = 1; i < ans.size(); i ++) {
      |                   ^
a.cc:29:27: error: request for member 'size' in 'ans', which is of non-class type 'std::vector<int>()'
   29 |   for (int i = 1; i < ans.size(); i ++) {
      |                           ^~~~