Language: C++

//include
//------------------------------------------
#include <map>
#include <iostream>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1000000007;

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int main() {
	LL ans = 1;
	int N;
	cin >> N;
	map<int,int> mp;
	for (int i = 0; i < N; i++) {
		int tmp;
		cin >> tmp;
		mp[tmp]++;
	}

	bool flag = true;
	int i = 0;
	if(N % 2 == 1) {
		mp[0]++;
		for (auto&& var : mp) {
			if(var.first != i || var.second != 2 || var.first > N) {
				flag = false;
				break;
			}
			i += 2;
		}
	}
	else {
		i = 1;
		for (auto&& var : mp) {
			if (var.first != i || var.second != 2 || var.first > N) {
				flag = false;
				break;
			}
			i += 2;
		}
	}

	if (flag) {
		for (int i = 0; i < N/2; i++) {
			ans = ans*2;
			ans = ans % MOD;
		}
		cout << ans << endl;
	}

	else {
		cout << 0 << endl;
	}

	return 0;
}


Compiler output:
a.cc: In function 'int toInt(std::string)':
a.cc:10:55: error: variable 'std::istringstream sin' has initializer but incomplete type
   10 | inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
      |                                                       ^
a.cc: In function 'std::string toString(T)':
a.cc:11:62: error: 'sout' has incomplete type
   11 | template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
      |                                                              ^~~~
In file included from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:4:
/usr/include/c++/14/iosfwd:106:11: note: declaration of 'std::ostringstream' {aka 'class std::__cxx11::basic_ostringstream<char>'}
  106 |     class basic_ostringstream;
      |           ^~~~~~~~~~~~~~~~~~~
a.cc: At global scope:
a.cc:19:9: error: 'vector' does not name a type
   19 | typedef vector<int> VI;
      |         ^~~~~~
a.cc:20:9: error: 'vector' does not name a type
   20 | typedef vector<VI> VVI;
      |         ^~~~~~
a.cc:21:9: error: 'vector' does not name a type
   21 | typedef vector<string> VS;
      |         ^~~~~~
a.cc:44:20: error: 'acos' was not declared in this scope
   44 | const double PI  = acos(-1.0);
      |                    ^~~~