Problem p02616 (C++)

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

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define MP make_pair
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
  cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
  int x12345;                                                                  \
  cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
const long long MOD = 1e9 + 7;
///////////////////////////////////////////////
// for template
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;
}
///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|

template <int MOD> struct Fp {
  long long val;

  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += 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 = val % MOD;

    if (val < 0)
      val += MOD * ((-val + MOD - 1) / MOD);

    return *this;
  }

  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD * ((-val + MOD - 1) / MOD);
    val = 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 istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> 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;
  }
};

using mint = Fp<MOD>;

int main() {
  ll N, K;
  cin >> N >> K;
  vector<ll> po, ng;
  ll zero = 0;
  REP(i, N) {
    ll a;
    cin >> a;
    if (a == 0)
      zero++;
    if (a >= 0) {
      po.emplace_back(a);
    } else if (a < 0) {
      ng.emplace_back(a);
    }
  }
  ll pnum = po.size();
  ll nnum = ng.size();

  sort(po.rbegin(), po.rend());
  sort(ALL(ng));

  int pp = 0;
  int np = 0;

  if (N == K) {
    ll ans = 1;
    REP(i, pnum) {
      ans = (ans * po[i]) % MOD;
      if (ans < 0)
        ans += MOD;
    }

    REP(j, nnum) {
      ans = (ans * ng[j]) % MOD;
      if (ans < 0)
        ans += MOD;
    }
    cout << ans << endl;
    return 0;
  }

  while (pp + np < K) {
    if ((pp + np < K - 1) and (pp >= pnum)) {
      np += 2;
    } else if ((pp + np == K - 1) or (np + 1 >= nnum)) {
      pp += 1;
      // if (pp > pnum){
      //     ll ans = 1;
      //     REP(i,K){
      //         ans = (ans * ng[nnum - i - 1]) % MOD;
      //         if (ans < 0) ans += MOD;
      //         if (ans == 0){
      //             cout << "i = " << i << endl;
      //         }
      //     }
      //     cout << ans << endl;
      //     return 0;
      // }
    } else {
      if (po[pp] * po[pp + 1] >= ng[np] * ng[np + 1]) {
        pp += 1;
      } else {
        np += 2;
      }
    }
  }

  ll ans = 1;
  REP(i, pp) { ans = (ans * po[i]) % MOD; }

  REP(j, np) {
    ans = (ans * ng[j]) % MOD;
    if (ans < 0)
      ans += MOD;
  }

  cout << ans << endl;
}

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

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define MP make_pair
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
  cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
  int x12345;                                                                  \
  cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
const long long MOD = 1e9 + 7;
///////////////////////////////////////////////
// for template
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;
}
///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|

template <int MOD> struct Fp {
  long long val;

  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += 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 = val % MOD;

    if (val < 0)
      val += MOD * ((-val + MOD - 1) / MOD);

    return *this;
  }

  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD * ((-val + MOD - 1) / MOD);
    val = 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 istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> 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;
  }
};

using mint = Fp<MOD>;

int main() {
  ll N, K;
  cin >> N >> K;
  vector<ll> po, ng;
  ll zero = 0;
  REP(i, N) {
    ll a;
    cin >> a;
    if (a == 0)
      zero++;
    if (a >= 0) {
      po.emplace_back(a);
    } else if (a < 0) {
      ng.emplace_back(a);
    }
  }
  ll pnum = po.size();
  ll nnum = ng.size();

  sort(po.rbegin(), po.rend());
  sort(ALL(ng));

  int pp = 0;
  int np = 0;

  if (N == K) {
    ll ans = 1;
    REP(i, pnum) {
      ans = (ans * po[i]) % MOD;
      if (ans < 0)
        ans += MOD;
    }

    REP(j, nnum) {
      ans = (ans * ng[j]) % MOD;
      if (ans < 0)
        ans += MOD;
    }
    cout << ans << endl;
    return 0;
  }

  while (pp + np < K) {
    if ((pp + np < K - 1) and (pp >= pnum)) {
      np += 2;
    } else if ((pp + np == K - 1) or (np + 1 >= nnum)) {
      pp += 1;
      if (pp > pnum) { // N != K
        ll ans = 1;
        REP(i, K) {
          ans = (ans * ng[nnum - i - 1]) % MOD;
          if (ans < 0)
            ans += MOD;
          if (ans == 0) {
            cout << "i = " << i << endl;
          }
        }
        cout << ans << endl;
        return 0;
      }
    } else {
      if (po[pp] * po[pp + 1] >= ng[np] * ng[np + 1]) {
        pp += 1;
      } else {
        np += 2;
      }
    }
  }

  ll ans = 1;
  REP(i, pp) { ans = (ans * po[i]) % MOD; }

  REP(j, np) {
    ans = (ans * ng[j]) % MOD;
    if (ans < 0)
      ans += MOD;
  }

  cout << ans << endl;
}