Problem p02616 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
// const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

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>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

template <typename T = int64> vector<T> make_v(size_t a) {
  return vector<T>(a);
}

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

template <typename F> struct FixPoint : F {
  FixPoint(F &&f) : F(forward<F>(f)) {}

  template <typename... Args> decltype(auto) operator()(Args &&...args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template <typename F> inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt<mod>;

int main() {
  int N, K;
  cin >> N >> K;
  vector<int64> A(N);
  cin >> A;

  vector<pair<int64, bool>> x;
  for (auto &p : A) {
    if (p > 0)
      x.emplace_back(p, true);
    else if (p < 0)
      x.emplace_back(-p, false);
    else
      x.emplace_back(0, false);
  }
  sort(x.rbegin(), x.rend());
  modint ret = 1;
  bool f = false;
  for (int i = 0; i < K; i++) {
    if (!x[i].second)
      f ^= 1;
    if (x[i].first == 0)
      f = false;
    ret *= x[i].first;
  }

  if (!f) {
    cout << ret << "\n";
    return 0;
  }

  int last_pos = -1, last_pos2 = -1;
  for (int i = 0; i < K; i++) {
    if (!x[i].second) { // -
      last_pos = x[i].first;
    } else { // +
      last_pos2 = x[i].first;
    }
  }
  // tap: - を外して + を入れる
  // ris: + を外して - を入れる
  int64 tap = 0, ris = 0;
  for (int i = K; i < N; i++) {
    if (x[i].second)
      chmax(tap, x[i].first); // +
    else
      chmax(ris, x[i].first); // -
  }

  if (last_pos2 > 0 && ris > 0) {
    if (last_pos > 0 && tap > 0) {
      int64 u = last_pos2 - ris;
      int64 v = last_pos - tap;
      if (u < v) {
        ret /= last_pos2;
        ret *= ris;
        cout << ret << "\n";
      } else {
        ret /= last_pos;
        ret *= tap;
        cout << ret << "\n";
        assert(0);
      }
    } else {
      ret /= last_pos2;
      ret *= ris;
      cout << ret << "\n";
    }
  } else {
    if (last_pos == 0 || tap == 0) {
      modint mul = 1;
      reverse(begin(x), end(x));
      for (int i = 0; i < K; i++) {
        if (x[i].second)
          mul *= x[i].first;
        else
          mul *= -x[i].first;
      }
      cout << mul << "\n";
    } else {
      ret /= last_pos;
      ret *= tap;
      cout << ret << "\n";
    }
  }
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
// const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

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>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

template <typename T = int64> vector<T> make_v(size_t a) {
  return vector<T>(a);
}

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

template <typename F> struct FixPoint : F {
  FixPoint(F &&f) : F(forward<F>(f)) {}

  template <typename... Args> decltype(auto) operator()(Args &&...args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template <typename F> inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt<mod>;

int main() {
  int N, K;
  cin >> N >> K;
  vector<int64> A(N);
  cin >> A;

  vector<pair<int64, bool>> x;
  for (auto &p : A) {
    if (p > 0)
      x.emplace_back(p, true);
    else if (p < 0)
      x.emplace_back(-p, false);
    else
      x.emplace_back(0, false);
  }
  sort(x.rbegin(), x.rend());
  modint ret = 1;
  bool f = false;
  for (int i = 0; i < K; i++) {
    if (!x[i].second)
      f ^= 1;
    if (x[i].first == 0)
      f = false;
    ret *= x[i].first;
  }

  if (!f) {
    cout << ret << "\n";
    return 0;
  }

  int last_pos = -1, last_pos2 = -1;
  for (int i = 0; i < K; i++) {
    if (!x[i].second) { // -
      last_pos = x[i].first;
    } else { // +
      last_pos2 = x[i].first;
    }
  }
  // tap: - を外して + を入れる
  // ris: + を外して - を入れる
  int64 tap = 0, ris = 0;
  for (int i = K; i < N; i++) {
    if (x[i].second)
      chmax(tap, x[i].first); // +
    else
      chmax(ris, x[i].first); // -
  }

  if (last_pos2 > 0 && ris > 0) {
    if (last_pos > 0 && tap > 0) {
      int64 u = last_pos2 - ris;
      int64 v = last_pos - tap;
      if (u < v) {
        ret /= last_pos2;
        ret *= ris;
        cout << ret << "\n";
      } else {
        random_device rng;
        if (rng() % 2) {
          ret /= last_pos2;
          ret *= ris;
          cout << ret << "\n";
        } else {
          ret /= last_pos;
          ret *= tap;
          cout << ret << "\n";
        }
      }
    } else {
      ret /= last_pos2;
      ret *= ris;
      cout << ret << "\n";
    }
  } else {
    if (last_pos == 0 || tap == 0) {
      modint mul = 1;
      reverse(begin(x), end(x));
      for (int i = 0; i < K; i++) {
        if (x[i].second)
          mul *= x[i].first;
        else
          mul *= -x[i].first;
      }
      cout << mul << "\n";
    } else {
      ret /= last_pos;
      ret *= tap;
      cout << ret << "\n";
    }
  }
}