Language: C++

#include <bits/stdc++.h>
using namespace std;
using ll=long long;
const ll MOD=1000000007;
const double PI=3.14159265358979;
const ll INF= pow(10,18);
typedef pair<ll,ll> P;
typedef vector<ll> vl;
typedef vector<vl> vvl;
#define FOR(i,a,b) for(ll i=a;i<b;i++)
#define rep(i,n) FOR(i,0,n)
string abc="abcdefghijklmnopqrstuvwxyz";
string ABC="ABCDEFGHIJKLMNOPQRSTUVWXYZ";

ll modpow(ll n,ll a){
  if(a==0){
    return 0;
  }
  else if(a%2==0){
    ll t=modpow(n,a/2);
    return t*t%MOD;
  }
  else
    return n*modpow(n,a-1)%MDO;
}
    
int main() {
  ll n;
  cin >> n;
  vl a(n);
  vl cnt(n);
  rep(i,n){
    cin >> a[i];
    ll cnt[a[i]]++;
  }
  bool a=false;
  ll i=n-1;
  while(i>=0){
    if(i!=0&&cnt[i]!=2){
      a=true;
    }
    if(i==0&&cnt[i]!=1){
      a=true;
    }
    i -=2
  }
  if(a){
    cout << 0 << endl;
  }
  else{
    cout << modpow(2,n/2) << endl;
  }
}

Compiler output:
a.cc: In function 'll modpow(ll, ll)':
a.cc:24:28: error: 'MDO' was not declared in this scope; did you mean 'MOD'?
   24 |     return n*modpow(n,a-1)%MDO;
      |                            ^~~
      |                            MOD
a.cc: In function 'int main()':
a.cc:34:17: error: expected initializer before '++' token
   34 |     ll cnt[a[i]]++;
      |                 ^~
a.cc:36:8: error: conflicting declaration 'bool a'
   36 |   bool a=false;
      |        ^
a.cc:30:6: note: previous declaration as 'vl a'
   30 |   vl a(n);
      |      ^
a.cc:40:9: error: no match for 'operator=' (operand types are 'vl' {aka 'std::vector<long long int>'} and 'bool')
   40 |       a=true;
      |         ^~~~
In file included from /usr/include/c++/14/vector:72,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/vector.tcc:210:5: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = long long int; _Alloc = std::allocator<long long int>]'
  210 |     vector<_Tp, _Alloc>::
      |     ^~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:211:42: note:   no known conversion for argument 1 from 'bool' to 'const std::vector<long long int>&'
  211 |     operator=(const vector<_Tp, _Alloc>& __x)
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/14/vector:66:
/usr/include/c++/14/bits/stl_vector.h:766:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = long long int; _Alloc = std::allocator<long long int>]'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:766:26: note:   no known conversion for argument 1 from 'bool' to 'std::vector<long long int>&&'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |                 ~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:788:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::initializer_list<_Tp>) [with _Tp = long long int; _Alloc = std::allocator<long long int>]'
  788 |       operator=(initializer_list<value_type> __l)
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:788:46: note:   no known conversion for argument 1 from 'bool' to 'std::initializer_list<long long int>'
  788 |       operator=(initializer_list<value_type> __l)
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
a.cc:43:9: error: no match for 'operator=' (operand types are 'vl' {aka 'std::vector<long long int>'} and 'bool')
   43 |       a=true;
      |         ^~~~
/usr/include/c++/14/bits/vector.tcc:210:5: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = long long int; _Alloc = std::allocator<long long int>]'
  210 |     vector<_Tp, _Alloc>::
      |     ^~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:211:42: note:   no known conversion for argument 1 from 'bool' to 'const std::vector<long long int>&'
  211 |     operator=(const vector<_Tp, _Alloc>& __x)
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:766:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = long long int; _Alloc = std::allocator<long long int>]'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:766:26: note:   no known conversion for argument 1 from 'bool' to 'std::vector<long long int>&&'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |                 ~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:788:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::initializer_list<_Tp>) [with _Tp = long long int; _Alloc = std::allocator<long long int>]'
  788 |       operator=(initializer_list<value_type> __l)
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:788:46: note:   no known conversion for argument 1 from 'bool' to 'std::initializer_list<long long int>'
  788 |       operator=(initializer_list<value_type> __l)
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
a.cc:45:10: error: expected ';' before '}' token
   45 |     i -=2
      |          ^
      |          ;
   46 |   }
      |   ~       
a.cc:47:6: error: could not convert 'a' from 'vl' {aka 'std::vector<long long int>'} to 'bool'
   47 |   if(a){
      |      ^
      |      |
      |      vl {aka std::vector<long long int>}