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

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto &(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

// modint
template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

// binomial coefficient
template <class T> struct BiCoef {
  vector<T> fact_, inv_, finv_;
  constexpr BiCoef() {}
  constexpr BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) {
    init(n);
  }
  constexpr void init(int n) noexcept {
    fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1);
    int MOD = fact_[0].getmod();
    for (int i = 2; i < n; i++) {
      fact_[i] = fact_[i - 1] * i;
      inv_[i] = -inv_[MOD % i] * (MOD / i);
      finv_[i] = finv_[i - 1] * inv_[i];
    }
  }
  constexpr T com(int n, int k) const noexcept {
    if (n < k || n < 0 || k < 0)
      return 0;
    if (n <= (int)fact_.size() - 1) {
      return fact_[n] * finv_[k] * finv_[n - k];
    }
    // C(n,k), n <= 1e9, k <= 1e6
    T res = 1;
    for (int i = n; i >= n - k + 1; i--) {
      T a = i;
      res *= a;
    }
    res *= finv_[k];
    return res;
  }
  constexpr T fact(int n) const noexcept {
    if (n < 0)
      return 0;
    return fact_[n];
  }
  constexpr T inv(int n) const noexcept {
    if (n < 0)
      return 0;
    return inv_[n];
  }
  constexpr T finv(int n) const noexcept {
    if (n < 0)
      return 0;
    return finv_[n];
  }
};

const int MOD = 1000000007;
// const int MOD = 998244353;
using mint = Fp<MOD>;
BiCoef<mint> bc;

ll calc(const vector<ll> &a) {
  ll n = SZ(a);
  if (n < 2) {
    return 0;
  }
  return a[n - 1] * a[n - 2];
}

int main() {
  ll n, k;
  cin >> n >> k;
  vector<ll> a(n), pos, neg;
  REP(i, n) {
    cin >> a[i];
    if (a[i] >= 0) {
      pos.push_back(a[i]);
    } else {
      neg.push_back(-a[i]);
    }
  }
  sort(a.rbegin(), a.rend());

  mint ans = 1;
  if (n == k || SZ(pos) == 0 && k % 2 == 1) {
    REP(i, k) { ans *= a[i]; }
    cout << ans << endl;
    return 0;
  }

  if (SZ(pos) % 2 == 1) {
    pos.push_back(0);
  }
  if (SZ(neg) % 2 == 1) {
    neg.push_back(0);
  }

  sort(ALL(pos));
  sort(ALL(neg));

  if (k % 2 == 1) {
    ans *= pos.back();
    pos.pop_back();
    k--;
  }

  REP(_, k / 2) {
    ll x = calc(pos), y = calc(neg);
    if (x <= y) {
      ans *= y;
      if (y == 0) {
        continue;
      }
      neg.pop_back();
      neg.pop_back();
    } else {
      ans *= x;
      if (x == 0) {
        continue;
      }
      pos.pop_back();
      pos.pop_back();
    }
  }
  cout << ans << endl;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto &(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

// modint
template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

// binomial coefficient
template <class T> struct BiCoef {
  vector<T> fact_, inv_, finv_;
  constexpr BiCoef() {}
  constexpr BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) {
    init(n);
  }
  constexpr void init(int n) noexcept {
    fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1);
    int MOD = fact_[0].getmod();
    for (int i = 2; i < n; i++) {
      fact_[i] = fact_[i - 1] * i;
      inv_[i] = -inv_[MOD % i] * (MOD / i);
      finv_[i] = finv_[i - 1] * inv_[i];
    }
  }
  constexpr T com(int n, int k) const noexcept {
    if (n < k || n < 0 || k < 0)
      return 0;
    if (n <= (int)fact_.size() - 1) {
      return fact_[n] * finv_[k] * finv_[n - k];
    }
    // C(n,k), n <= 1e9, k <= 1e6
    T res = 1;
    for (int i = n; i >= n - k + 1; i--) {
      T a = i;
      res *= a;
    }
    res *= finv_[k];
    return res;
  }
  constexpr T fact(int n) const noexcept {
    if (n < 0)
      return 0;
    return fact_[n];
  }
  constexpr T inv(int n) const noexcept {
    if (n < 0)
      return 0;
    return inv_[n];
  }
  constexpr T finv(int n) const noexcept {
    if (n < 0)
      return 0;
    return finv_[n];
  }
};

const int MOD = 1000000007;
// const int MOD = 998244353;
using mint = Fp<MOD>;
BiCoef<mint> bc;

ll calc(const vector<ll> &a) {
  ll n = SZ(a);
  return a[n - 1] * a[n - 2];
}

int main() {
  ll n, k;
  cin >> n >> k;
  vector<ll> a(n), pos, neg;
  REP(i, n) {
    cin >> a[i];
    if (a[i] >= 0) {
      pos.push_back(a[i]);
    } else {
      neg.push_back(-a[i]);
    }
  }
  sort(a.rbegin(), a.rend());

  mint ans = 1;
  if (n == k || SZ(pos) == 0 && k % 2 == 1) {
    REP(i, k) { ans *= a[i]; }
    cout << ans << endl;
    return 0;
  }

  if (SZ(pos) % 2 == 1) {
    pos.push_back(0);
  }
  if (SZ(neg) % 2 == 1) {
    neg.push_back(0);
  }

  sort(ALL(pos));
  sort(ALL(neg));

  if (k % 2 == 1) {
    ans *= pos.back();
    pos.pop_back();
    k--;
  }

  REP(_, k / 2) {
    ll x = calc(pos), y = calc(neg);
    if (x <= y) {
      ans *= y;
      neg.pop_back();
      neg.pop_back();
    } else {
      ans *= x;
      pos.pop_back();
      pos.pop_back();
    }
  }
  cout << ans << endl;
}