Problem p02616 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include <boost/multiprecision/cpp_int.hpp>
// using i128 = boost::multiprecision::int128_t;
#define _GLIBCXX_DEBUG
using namespace std;
using ll = long long;
using ld = long double;
using V = vector<int>;
using Vll = vector<ll>;
using Vld = vector<ld>;
using Vbo = vector<bool>;
using VV = vector<V>;
using VVll = vector<Vll>;
using VVld = vector<Vld>;
using VVbo = vector<Vbo>;
using VVV = vector<VV>;
using VVVll = vector<VVll>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using Pld = pair<ld, ld>;
#define rep2(i, m, n) for (int i = int(m); i < int(n); ++i)
#define drep2(i, m, n) for (int i = int(m) - 1; i >= int(n); --i)
#define rep(i, n) rep2(i, 0, n)
#define drep(i, n) drep2(i, n, 0)
#define all(a) a.begin(), a.end()
struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T> inline int sz(T &x) { return x.size(); }
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << '(' << p.first << ", " << p.second << ')';
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &e : v)
    os << e << ' ';
  return os;
}
template <typename T> inline int count_between(vector<T> &a, T l, T r) {
  return lower_bound(all(a), r) - lower_bound(all(a), l);
} // [l, r)
inline int fLog2(const int x) {
  return 31 - __builtin_clz(x);
} // floor(log2(x))
inline int fLog2(const ll x) { return 63 - __builtin_clzll(x); }
inline int cLog2(const int x) {
  return (x == 1) ? 0 : 32 - __builtin_clz(x - 1);
} // ceil(log2(x))
inline int cLog2(const ll x) {
  return (x == 1) ? 0 : 64 - __builtin_clzll(x - 1);
}
inline int popcount(const int x) { return __builtin_popcount(x); }
inline int popcount(const ll x) { return __builtin_popcountll(x); }
const int INF = 1 << 30;
const ll INFll = 1ll << 62;
const ld EPS = 1e-10;
const ld PI = acos(-1.0);
const int MOD = int(1e9) + 7;
// const int MOD  = 998244353;

struct mint {
  ll x;
  mint(ll x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x -= a.x) < 0)
      x += MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  mint &operator/=(const mint r) {
    ll a = r.x, b = MOD, u = 1, v = 0;
    while (b) {
      ll t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    x = x * u % MOD;
    if (x < 0)
      x += MOD;
    return *this;
  }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
// istream& operator>>(istream& is, mint& a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
using Vm = vector<mint>;
using VVm = vector<Vm>;
using VVVm = vector<VVm>;

int main() {
  ll n, k;
  cin >> n >> k;
  Vll a(n);
  cin >> a;
  sort(all(a), [](int i, int j) -> bool { return abs(i) > abs(j); });

  VVll b(2);
  VV cnt(2, V(n + 1));
  rep(i, n) {
    if (a[i] >= 0)
      b[0].push_back(a[i]);
    else
      b[1].push_back(-a[i]);
    cnt[0][i + 1] = cnt[0][i] + (a[i] >= 0);
    cnt[1][i + 1] = cnt[1][i] + (a[i] < 0);
  }

  if (n == k) {
    mint ans = 1;
    rep(i, n) ans *= mint(a[i]);
    cout << ans << '\n';
    return 0;
  }

  if (cnt[0][n] == 0) {
    mint ans = 1;
    if (k & 1) {
      reverse(all(b[1]));
      ans *= -1;
    }
    rep(i, k) ans *= b[1][i];
    cout << ans << '\n';
    return 0;
  }

  if (!(cnt[1][k] & 1)) {
    mint ans = 1;
    rep(i, k) ans *= abs(a[i]);
    cout << ans << '\n';
    return 0;
  }

  if (cnt[0][k] == 0) {
    mint ans = b[0][0];
    rep(i, k - 1) ans *= b[1][i];
    cout << ans << '\n';
    return 0;
  }

  ll now = 0;
  rep(j, 2) if (cnt[j][k] < cnt[j][n])
      chmax(now, b[j][cnt[j][k] - 1] * b[j][cnt[j][k]]);
  mint ans = now;
  rep(j, 2) rep(i, cnt[j][k] - 1) ans *= b[0][i];
  cout << ans << '\n';
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
// #include <boost/multiprecision/cpp_int.hpp>
// using i128 = boost::multiprecision::int128_t;
#define _GLIBCXX_DEBUG
using namespace std;
using ll = long long;
using ld = long double;
using V = vector<int>;
using Vll = vector<ll>;
using Vld = vector<ld>;
using Vbo = vector<bool>;
using VV = vector<V>;
using VVll = vector<Vll>;
using VVld = vector<Vld>;
using VVbo = vector<Vbo>;
using VVV = vector<VV>;
using VVVll = vector<VVll>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using Pld = pair<ld, ld>;
#define rep2(i, m, n) for (int i = int(m); i < int(n); ++i)
#define drep2(i, m, n) for (int i = int(m) - 1; i >= int(n); --i)
#define rep(i, n) rep2(i, 0, n)
#define drep(i, n) drep2(i, n, 0)
#define all(a) a.begin(), a.end()
struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T> inline int sz(T &x) { return x.size(); }
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << '(' << p.first << ", " << p.second << ')';
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &e : v)
    os << e << ' ';
  return os;
}
template <typename T> inline int count_between(vector<T> &a, T l, T r) {
  return lower_bound(all(a), r) - lower_bound(all(a), l);
} // [l, r)
inline int fLog2(const int x) {
  return 31 - __builtin_clz(x);
} // floor(log2(x))
inline int fLog2(const ll x) { return 63 - __builtin_clzll(x); }
inline int cLog2(const int x) {
  return (x == 1) ? 0 : 32 - __builtin_clz(x - 1);
} // ceil(log2(x))
inline int cLog2(const ll x) {
  return (x == 1) ? 0 : 64 - __builtin_clzll(x - 1);
}
inline int popcount(const int x) { return __builtin_popcount(x); }
inline int popcount(const ll x) { return __builtin_popcountll(x); }
const int INF = 1 << 30;
const ll INFll = 1ll << 62;
const ld EPS = 1e-10;
const ld PI = acos(-1.0);
const int MOD = int(1e9) + 7;
// const int MOD  = 998244353;

struct mint {
  ll x;
  mint(ll x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x -= a.x) < 0)
      x += MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  mint &operator/=(const mint r) {
    ll a = r.x, b = MOD, u = 1, v = 0;
    while (b) {
      ll t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    x = x * u % MOD;
    if (x < 0)
      x += MOD;
    return *this;
  }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
// istream& operator>>(istream& is, mint& a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
using Vm = vector<mint>;
using VVm = vector<Vm>;
using VVVm = vector<VVm>;

int main() {
  ll n, k;
  cin >> n >> k;
  Vll a(n);
  cin >> a;
  sort(all(a), [](int i, int j) -> bool { return abs(i) > abs(j); });

  VVll b(2);
  VV cnt(2, V(n + 1));
  rep(i, n) {
    if (a[i] >= 0)
      b[0].push_back(a[i]);
    else
      b[1].push_back(-a[i]);
    cnt[0][i + 1] = cnt[0][i] + (a[i] >= 0);
    cnt[1][i + 1] = cnt[1][i] + (a[i] < 0);
  }

  if (n == k) {
    mint ans = 1;
    rep(i, n) ans *= mint(a[i]);
    cout << ans << '\n';
    return 0;
  }

  if (cnt[0][n] == 0) {
    mint ans = 1;
    if (k & 1) {
      reverse(all(b[1]));
      ans *= -1;
    }
    rep(i, k) ans *= b[1][i];
    cout << ans << '\n';
    return 0;
  }

  if (!(cnt[1][k] & 1)) {
    mint ans = 1;
    rep(i, k) ans *= abs(a[i]);
    cout << ans << '\n';
    return 0;
  }

  if (cnt[0][k] == 0) {
    mint ans = b[0][0];
    rep(i, k - 1) ans *= b[1][i];
    cout << ans << '\n';
    return 0;
  }

  ll now = 0;
  rep(j, 2) if (cnt[j][k] < cnt[j][n])
      chmax(now, b[j][cnt[j][k] - 1] * b[j][cnt[j][k]]);
  mint ans = now;
  rep(j, 2) rep(i, cnt[j][k] - 1) ans *= b[j][i];
  cout << ans << '\n';
  return 0;
}