Problem p03201 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define p(s) std::cout << s;
#define pl(s) std::cout << s << endl;
#define all(v) v.begin(), v.end()
#define showVector(v)                                                          \
  REP(i, v.size()) {                                                           \
    p(v[i]);                                                                   \
    p(" ")                                                                     \
  }                                                                            \
  pl("")
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
typedef long long int ll;
typedef pair<ll, ll> P_ii;
typedef pair<double, double> P_dd;

template <class T> vector<T> make_vec(size_t a) { return vector<T>(a); }

template <class T, class... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

ll gcd(ll a, ll b) {
  if (a < b)
    swap(a, b);

  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return (a / g) * b;
}

// 素数判定 O(√n)
bool is_prime(int n) {
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}

// 約数列挙 O(√n)
vector<ll> divisor(ll n) {
  vector<ll> res;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      res.push_back(i);
      if (i != n / i)
        res.push_back(n / i);
    }
  }
  return res;
}

vector<pair<ll, ll>> prime_factorize(ll n) {
  vector<pair<ll, ll>> res;
  for (ll p = 2; p * p <= n; ++p) {
    if (n % p != 0)
      continue;
    ll num = 0;
    while (n % p == 0) {
      ++num;
      n /= p;
    }
    res.push_back(make_pair(p, num));
  }
  if (n != 1)
    res.push_back(make_pair(n, 1));
  return res;
}

// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x(x % mod) {}
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

const int MOD = 1000000007;
const int inf = 1e9 + 7;
const ll longinf = 1LL << 60;

void addM(ll &a, ll b) {
  a += b;
  if (a >= MOD)
    a -= MOD;
}

void mulM(ll &a, ll b) { a = ((a % MOD) * (b % MOD)) % MOD; }

ll powM(ll a, ll b) {
  ll ret = 1;
  ll tmp = a;
  while (b > 0) {
    if ((b & 1) == 1)
      ret = (ret * tmp) % MOD;
    tmp = (tmp * tmp) % MOD;
    b = b >> 1;
  }
  return ret;
}

int get2beki(int a) {
  int ret = 1;
  while (ret <= a)
    ret *= 2;
  return ret;
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  cin >> N;

  multiset<ll> st;

  vector<int> v(N);
  REP(i, N) {
    cin >> v[i];
    st.insert(v[i]);
  }
  sort(all(v));

  int ans = 0;
  for (int i = N - 1; i >= 0; i--) {
    if (!st.count(v[i]))
      continue;

    auto ite = st.find(v[i]);
    st.erase(ite);

    int b = get2beki(v[i]) - v[i];

    if (!st.count(b))
      continue;
    auto it = st.find(b);

    if (*it == b) {
      ans++;
      st.erase(it);
    }
  }
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define p(s) std::cout << s;
#define pl(s) std::cout << s << endl;
#define all(v) v.begin(), v.end()
#define showVector(v)                                                          \
  REP(i, v.size()) {                                                           \
    p(v[i]);                                                                   \
    p(" ")                                                                     \
  }                                                                            \
  pl("")
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
typedef long long int ll;
typedef pair<ll, ll> P_ii;
typedef pair<double, double> P_dd;

template <class T> vector<T> make_vec(size_t a) { return vector<T>(a); }

template <class T, class... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

ll gcd(ll a, ll b) {
  if (a < b)
    swap(a, b);

  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return (a / g) * b;
}

// 素数判定 O(√n)
bool is_prime(int n) {
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}

// 約数列挙 O(√n)
vector<ll> divisor(ll n) {
  vector<ll> res;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      res.push_back(i);
      if (i != n / i)
        res.push_back(n / i);
    }
  }
  return res;
}

vector<pair<ll, ll>> prime_factorize(ll n) {
  vector<pair<ll, ll>> res;
  for (ll p = 2; p * p <= n; ++p) {
    if (n % p != 0)
      continue;
    ll num = 0;
    while (n % p == 0) {
      ++num;
      n /= p;
    }
    res.push_back(make_pair(p, num));
  }
  if (n != 1)
    res.push_back(make_pair(n, 1));
  return res;
}

// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x(x % mod) {}
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

const int MOD = 1000000007;
const int inf = 1e9 + 7;
const ll longinf = 1LL << 60;

void addM(ll &a, ll b) {
  a += b;
  if (a >= MOD)
    a -= MOD;
}

void mulM(ll &a, ll b) { a = ((a % MOD) * (b % MOD)) % MOD; }

ll powM(ll a, ll b) {
  ll ret = 1;
  ll tmp = a;
  while (b > 0) {
    if ((b & 1) == 1)
      ret = (ret * tmp) % MOD;
    tmp = (tmp * tmp) % MOD;
    b = b >> 1;
  }
  return ret;
}

int get2beki(int a) {
  int ret = 1;
  while (ret <= a)
    ret *= 2;
  return ret;
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  cin >> N;

  multiset<ll> st;

  vector<int> v(N);
  REP(i, N) {
    cin >> v[i];
    st.insert(v[i]);
  }
  sort(all(v));

  int ans = 0;
  for (int i = N - 1; i >= 0; i--) {
    if (!st.count(v[i]))
      continue;

    auto ite = st.find(v[i]);
    st.erase(ite);

    int b = get2beki(v[i]) - v[i];

    auto it = st.lower_bound(b);
    if (*it == b) {
      ans++;
      st.erase(it);
    }
  }
  cout << ans << endl;

  return 0;
}