Problem p03846 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORR(i, m, n) for (int i = m; i >= n; i--)
#define SORT(v, n) sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define INF 999999999
#define M_PI 3.14159265358979323846
#define ALL(X) (X).begin(), (X).end()

#ifdef __LOCAL
#define DBG(X) cout << #X << " = " << (X) << endl;
#define SAY(X) cout << (X) << endl;
#else
#define DBG(X)
#define SAY(X)
#endif

#ifdef __LOCAL
#include <filesystem>
namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

int dx[] = {1, 0, -1, 0, 0};
int dy[] = {0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));

std::string pad(int num) {
  char buffer[4];
  std::snprintf(buffer, sizeof(buffer), "%03d", num);
  return buffer;
}
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const map<T, S> p) {
  for (auto el : p)
    cout << "[" << el.first << "," << el.second << "] ";
  return os;
}
template <typename T>
inline ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto el : v)
    cout << el << " ";
  return os;
}
template <typename T> inline T fetch() {
  T ret;
  cin >> ret;
  return ret;
}
template <typename T> inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto &elem : ret)
    cin >> elem;
  return ret;
}

ll N;
vector<ll> A;
const ll MOD = (ll)(1e9 + 7);

void input() {
#ifdef __LOCAL
  fs::path path_to_file = __FILE__;
  fs::path input, output;
  input = output = path_to_file.parent_path();
  input += string("/input/") + string(path_to_file.stem()) + string(".txt");
  output += string("/output/") + string(path_to_file.stem()) + string(".txt");
  freopen(input.c_str(), "r", stdin);
  freopen(output.c_str(), "w", stdout);
#endif
  cin >> N;
  REP(i, N) {
    ll a;
    cin >> a;
    A.push_back(a);
  }
  VSORT(A)
}

bool isValid(vector<ll> a) {
  bool ans = true;
  if (N % 2 == 1) {
    ans &= (a[0] == 0);
    for (int i = 1; i <= ((N - 1) / 2); i++) {
      ans &= (a[2 * i - 1] == 2 * i);
      ans &= (a[2 * i] == 2 * i);
    }

  } else {
    for (int i = 0; i < N / 2; i++) {
      ans &= (a[2 * i] == 2 * i + 1);
      ans &= (a[2 * i + 1] == 2 * i + 1);
    }
  }
  return ans;
}

ll mod_pow(ll a, ll b) {
  ll res = 1LL;
  while (b > 0) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >> 1;
  }
  return res;
}

int solve() {
  if (!isValid(A)) {
    cout << 0 << endl;
    return 0;
  }
  ll ans = (N % 2 == 0) ? mod_pow(2LL, N / 2) : mod_pow(2LL, (N - 1) / 2);
  cout << ans << endl;
  return 0;
}

int main() {
  input();
  solve();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORR(i, m, n) for (int i = m; i >= n; i--)
#define SORT(v, n) sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define INF 999999999
#define M_PI 3.14159265358979323846
#define ALL(X) (X).begin(), (X).end()

#ifdef __LOCAL
#define DBG(X) cout << #X << " = " << (X) << endl;
#define SAY(X) cout << (X) << endl;
#else
#define DBG(X)
#define SAY(X)
#endif

#ifdef __LOCAL
#include <filesystem>
namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

int dx[] = {1, 0, -1, 0, 0};
int dy[] = {0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));

std::string pad(int num) {
  char buffer[4];
  std::snprintf(buffer, sizeof(buffer), "%03d", num);
  return buffer;
}
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const map<T, S> p) {
  for (auto el : p)
    cout << "[" << el.first << "," << el.second << "] ";
  return os;
}
template <typename T>
inline ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto el : v)
    cout << el << " ";
  return os;
}
template <typename T> inline T fetch() {
  T ret;
  cin >> ret;
  return ret;
}
template <typename T> inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto &elem : ret)
    cin >> elem;
  return ret;
}

ll N;
vector<ll> A;
const ll MOD = (ll)(1e9 + 7);

void input() {
#ifdef __LOCAL
  fs::path path_to_file = __FILE__;
  fs::path input, output;
  input = output = path_to_file.parent_path();
  input += string("/input/") + string(path_to_file.stem()) + string(".txt");
  output += string("/output/") + string(path_to_file.stem()) + string(".txt");
  freopen(input.c_str(), "r", stdin);
  freopen(output.c_str(), "w", stdout);
#endif
  cin >> N;
  REP(i, N) {
    ll a;
    cin >> a;
    A.push_back(a);
  }
  VSORT(A)
}

bool isValid(vector<ll> a) {
  bool ans = true;
  if (N % 2 == 1) {
    ans &= (a[0] == 0);
    for (int i = 1; i <= ((N - 1) / 2); i++) {
      ans &= (a[2 * i - 1] == 2 * i);
      ans &= (a[2 * i] == 2 * i);
    }

  } else {
    for (int i = 0; i < N / 2; i++) {
      ans &= (a[2 * i] == 2 * i + 1);
      ans &= (a[2 * i + 1] == 2 * i + 1);
    }
  }
  return ans;
}

ll mod_pow(ll a, ll b) {
  ll res = 1LL;
  while (b > 0) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b = (b >> 1);
  }
  return res;
}

int solve() {
  if (!isValid(A)) {
    cout << 0 << endl;
    return 0;
  }
  ll ans = (N % 2 == 0) ? mod_pow(2LL, N / 2) : mod_pow(2LL, (N - 1) / 2);
  cout << ans << endl;
  return 0;
}

int main() {
  input();
  solve();
  return 0;
}