Problem p02616 (C++) — buggy submission, verdict: Runtime Error

// includes
#include <bits/stdc++.h>
using namespace std;

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define irep(itr, st) for (auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for (auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define whole(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define bit(n) (1LL << (n))
#define F first
#define S second
// functions
template <typename T> void unique(T &c) {
  c.erase(std::unique(c.begin(), c.end()), c.end());
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 60;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(20);
  }
} fast_io_;

template <typename T> T power(T a, T n, T mod) {
  T res = 1;
  T tmp = n;
  T curr = a;
  while (tmp) {
    if (tmp % 2 == 1) {
      res = (T)(res * curr % mod);
    }
    curr = (T)(curr * curr % mod);
    tmp >>= 1;
  }

  return res;
}

template <typename T> T extgcd(T a, T b, T &x, T &y) {
  T d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1, y = 0;
  }
  return d;
}

template <typename T> T modinv(T a, T m) {
  long long x = 0, y = 0;
  extgcd<long long>(a, m, x, y);
  x %= m;
  if (x < 0)
    x += m;
  return x;
}

int calc(int n, int k, vector<ll> a) {
  vector<ll> pos, neg;
  int zero = 0;
  rep(i, n) {
    if (a[i] > 0)
      pos.pb(a[i]);
    else if (a[i] < 0)
      neg.pb(a[i]);
    else
      zero++;
  }
  sort(whole(pos));
  sort(whole(neg));
  if (k == n) {
    ll res = a[0];
    FOR(i, 1, n) res = res * a[i] % mod;
    if (res < 0)
      res += mod;
    cout << res << endl;
    return 0;
  }
  if (zero) {
    for (int i = 0; i <= sz(neg) && i <= k; i += 2) {
      if (i + sz(pos) >= k) {
        vector<ll> b;
        rep(j, n) {
          if (a[j] != 0)
            b.pb(a[j]);
        }
        calc(sz(b), k, b);
        return 0;
      }
    }
    cout << 0 << endl;
    return 0;
  }
  // minus
  if (sz(neg) == n) {
    if (k & 1)
      reverse(whole(neg));
    ll res = neg[0];
    FOR(i, 1, k) res = res * neg[i] % mod;
    if (res < 0)
      res += mod;
    cout << res << endl;
    return 0;
  }
  if (sz(pos) == n) {
    sort(whole(a));
    reverse(whole(a));
    ll res = a[0];
    FOR(i, 1, k) res = res * a[i] % mod;
    if (res < 0)
      res += mod;
    cout << res << endl;
    return 0;
  }

  ll res = 0;
  ll curr = 1;
  int j = 0, c = 0;
  reverse(whole(pos));
  if (k & 1) {
    curr = pos[0];
    j++;
    c++;
  }
  for (; j + 1 < k && j + 1 < sz(pos); j += 2) {
    curr = curr * pos[j] % mod;
    curr = curr * pos[j + 1] % mod;
    c += 2;
  }
  int npos = c;
  j = 0;
  while (c < k && j + 1 < sz(neg)) {
    curr = curr * neg[j] % mod;
    curr = curr * neg[j + 1] % mod;
    c += 2;
    j += 2;
  }
  res = curr;
  while (j + 1 < sz(neg) && npos > 1) {
    ll tmp1 = neg[j] * neg[j + 1];
    ll tmp2 = pos[npos - 1] * pos[npos - 2];
    if (tmp1 >= tmp2) {
      curr = curr * neg[j] % mod;
      curr = curr * neg[j + 1] % mod;
      curr = curr * modinv<ll>(pos[npos - 1], mod) % mod;
      curr = curr * modinv<ll>(pos[npos - 2], mod) % mod;
      if (curr < 0)
        curr += mod;
      npos -= 2;
      j += 2;
      res = curr;
    } else {
      break;
    }
  }

  cout << res << endl;
  return 0;
}

int main(int argc, char const *argv[]) {
  int n, k;
  cin >> n >> k;
  vector<ll> a(n);
  cin >> a;
  calc(n, k, a);
  return 0;
}


Error: 0

Fixed submission:

// includes
#include <bits/stdc++.h>
using namespace std;

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define irep(itr, st) for (auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for (auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define whole(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define bit(n) (1LL << (n))
#define F first
#define S second
// functions
template <typename T> void unique(T &c) {
  c.erase(std::unique(c.begin(), c.end()), c.end());
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 60;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(20);
  }
} fast_io_;

template <typename T> T power(T a, T n, T mod) {
  T res = 1;
  T tmp = n;
  T curr = a;
  while (tmp) {
    if (tmp % 2 == 1) {
      res = (T)(res * curr % mod);
    }
    curr = (T)(curr * curr % mod);
    tmp >>= 1;
  }

  return res;
}

template <typename T> T extgcd(T a, T b, T &x, T &y) {
  T d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1, y = 0;
  }
  return d;
}

template <typename T> T modinv(T a, T m) {
  long long x = 0, y = 0;
  extgcd<long long>(a, m, x, y);
  x %= m;
  if (x < 0)
    x += m;
  return x;
}

int main(int argc, char const *argv[]) {
  int n, k;
  cin >> n >> k;
  vector<ll> pos, neg;
  vector<ll> a(n);
  cin >> a;
  int zero = 0;
  rep(i, n) {
    if (a[i] > 0)
      pos.pb(a[i]);
    else if (a[i] < 0)
      neg.pb(a[i]);
    else
      zero++;
  }
  sort(whole(pos));
  sort(whole(neg));
  if (k == n) {
    ll res = a[0];
    FOR(i, 1, n) res = res * a[i] % mod;
    if (res < 0)
      res += mod;
    cout << res << endl;
    return 0;
  }
  // minus
  if (sz(neg) == n && k % 2 == 1) {
    reverse(whole(neg));
    ll res = neg[0];
    FOR(i, 1, k) res = res * neg[i] % mod;
    if (res < 0)
      res += mod;
    cout << res << endl;
    return 0;
  }
  // zero
  if (k > sz(pos) + sz(neg)) {
    cout << 0 << endl;
    return 0;
  }
  ll res = 0;
  ll curr = 1;
  int j = 0, c = 0;
  reverse(whole(pos));
  if (k & 1) {
    curr = pos[0];
    j++;
    c++;
  }
  for (; j + 1 < k && j + 1 < sz(pos); j += 2) {
    curr = curr * pos[j] % mod;
    curr = curr * pos[j + 1] % mod;
    c += 2;
  }
  int npos = c;
  j = 0;
  while (c < k && j + 1 < sz(neg)) {
    curr = curr * neg[j] % mod;
    curr = curr * neg[j + 1] % mod;
    c += 2;
    j += 2;
  }
  res = curr;
  while (j + 1 < sz(neg) && npos > 1) {
    ll tmp1 = neg[j] * neg[j + 1];
    ll tmp2 = pos[npos - 1] * pos[npos - 2];
    if (tmp1 >= tmp2) {
      curr = curr * neg[j] % mod;
      curr = curr * neg[j + 1] % mod;
      curr = curr * modinv<ll>(pos[npos - 1], mod) % mod;
      curr = curr * modinv<ll>(pos[npos - 2], mod) % mod;
      npos -= 2;
      j += 2;
      res = max(res, curr);
    } else {
      break;
    }
  }

  cout << res << endl;
  return 0;
}