Problem p02616 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using LL = long long;
using LD = long double;
using PII = pair<int, int>;
using PLI = pair<long long, int>;
using PLL = pair<long long, long long>;
using TI = tuple<int, int, int>;
mt19937 mrand(random_device{}());
int rnd(int x) { return mrand() % x; }
template <class A, class B>
ostream &operator<<(ostream &out, const pair<A, B> &p) {
  return out << "(" << p.first << ", " << p.second << ")";
}
template <typename T, size_t N>
ostream &operator<<(ostream &out, const array<T, N> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const deque<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename T>
ostream &operator<<(ostream &out, const unordered_set<T> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ", ");
    out << p.first << ":" << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << '\n';
  else
    cout << "NO" << '\n';
}
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << '\n';
  else
    cout << "No" << '\n';
}
template <class T> T gcd(T a, T b) {
  while (b) {
    T tmp = b;
    b = a % b;
    a = tmp;
  }
  return a;
}
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1> void __f(const char *name, Arg1 &&arg1) {
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}

long long qpow(long long a, long long b, int MOD) {
  if (b == 0)
    return 1;
  long long res = 1;
  while (b) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}

const int MOD = 1e9 + 7;
const int maxn = 2e6;
int fac[maxn + 5];
int invfac[maxn + 5];
void init() {
  fac[0] = 1;
  for (int i = 1; i <= maxn; i++)
    fac[i] = 1LL * fac[i - 1] * i % MOD;
  invfac[maxn] = (int)(qpow(fac[maxn], MOD - 2, MOD));
  for (int i = maxn - 1; i >= 0; i--)
    invfac[i] = 1LL * invfac[i + 1] * (i + 1) % MOD;
}
int C(int n, int k) {
  assert(0 <= k && n >= k);
  int tmp = (int)(1LL * invfac[k] * invfac[n - k] % MOD);
  return (int)(1LL * fac[n] * tmp % MOD);
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  int n, k;
  cin >> n >> k;
  vector<LL> a(n), pos, zero, neg;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (a[i] > 0) {
      pos.push_back(a[i]);
    } else if (a[i] == 0) {
      zero.push_back(a[i]);
    } else {
      neg.push_back(a[i]);
    }
  }
  sort(pos.begin(), pos.end());
  sort(neg.begin(), neg.end());
  reverse(neg.begin(), neg.end());
  int pcnt = pos.size(), zcnt = zero.size(), ncnt = neg.size();
  LL ALL = 1;
  for (auto &v : a)
    ALL = (ALL * v) % MOD;
  ALL = (ALL + MOD) % MOD;
  if (k == 1) {
    LL ans = *max_element(a.begin(), a.end()) % MOD;
    cout << (ans + MOD) % MOD << '\n';
    return 0;
  }
  if (k == pcnt + zcnt + ncnt) {
    cout << ALL << '\n';
    return 0;
  }
  if (k > pcnt + ncnt) {
    cout << 0 << '\n';
    return 0;
  } else if (pcnt == 0) {
    if (k & 1) {
      if (zcnt) {
        cout << 0 << '\n';
        return 0;
      }
      LL now = 1;
      for (int i = 0; i < k; i++)
        now = (now * neg[i]) % MOD;
      cout << (now + MOD) % MOD << '\n';
      return 0;
    } else {
      if (k > ncnt) {
        cout << 0 << '\n';
        return 0;
      }
      LL now = 1;
      for (int i = ncnt - 1; i >= ncnt - k; i--) {
        now = (now * neg[i]) % MOD;
      }
      cout << (now + MOD) % MOD << '\n';
      return 0;
    }
  } else {
    LL now = 1;
    int i = pos.size() - 1, j = neg.size() - 1;
    for (; k;) {
      if (k == 1) {
        if (i >= 0) {
          assert(0);
          now = now * pos[i--] % MOD;
        } else if (j >= 1) {
          now = now * qpow(pos[0], MOD - 2, MOD) % MOD;
          now = now * neg[j--] % MOD;
          now = now * neg[j--] % MOD;
        } else {
          if (zcnt)
            now = 0;
          else
            now = now * neg[j] % MOD;
        }
        k--;
      } else {
        LL P = 0, N = 0;
        if (i >= 1)
          P = pos[i] * pos[i - 1];
        else if (i == 0)
          P = pos[i];
        if (j >= 1)
          N = neg[j] * neg[j - 1];
        else if (j == 0)
          N = neg[j];
        if (P > N) {
          now = now * (P % MOD) % MOD;
          k -= 1 + (i > 0);
          i -= 1 + (i > 0);
        } else {
          now = now * (N % MOD) % MOD;
          k -= 1 + (j > 0);
          j -= 1 + (j > 0);
        }
      }
      // trace(k, now);
    }

    cout << (now + MOD) % MOD << '\n';
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using LL = long long;
using LD = long double;
using PII = pair<int, int>;
using PLI = pair<long long, int>;
using PLL = pair<long long, long long>;
using TI = tuple<int, int, int>;
mt19937 mrand(random_device{}());
int rnd(int x) { return mrand() % x; }
template <class A, class B>
ostream &operator<<(ostream &out, const pair<A, B> &p) {
  return out << "(" << p.first << ", " << p.second << ")";
}
template <typename T, size_t N>
ostream &operator<<(ostream &out, const array<T, N> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const deque<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename T>
ostream &operator<<(ostream &out, const unordered_set<T> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ", ");
    out << p.first << ":" << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << '\n';
  else
    cout << "NO" << '\n';
}
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << '\n';
  else
    cout << "No" << '\n';
}
template <class T> T gcd(T a, T b) {
  while (b) {
    T tmp = b;
    b = a % b;
    a = tmp;
  }
  return a;
}
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1> void __f(const char *name, Arg1 &&arg1) {
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}

long long qpow(long long a, long long b, int MOD) {
  if (b == 0)
    return 1;
  long long res = 1;
  while (b) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}

const int MOD = 1e9 + 7;
const int maxn = 2e6;
int fac[maxn + 5];
int invfac[maxn + 5];
void init() {
  fac[0] = 1;
  for (int i = 1; i <= maxn; i++)
    fac[i] = 1LL * fac[i - 1] * i % MOD;
  invfac[maxn] = (int)(qpow(fac[maxn], MOD - 2, MOD));
  for (int i = maxn - 1; i >= 0; i--)
    invfac[i] = 1LL * invfac[i + 1] * (i + 1) % MOD;
}
int C(int n, int k) {
  assert(0 <= k && n >= k);
  int tmp = (int)(1LL * invfac[k] * invfac[n - k] % MOD);
  return (int)(1LL * fac[n] * tmp % MOD);
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  int n, k;
  cin >> n >> k;
  vector<LL> a(n), pos, zero, neg;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (a[i] > 0) {
      pos.push_back(a[i]);
    } else if (a[i] == 0) {
      zero.push_back(a[i]);
    } else {
      neg.push_back(a[i]);
    }
  }
  sort(pos.begin(), pos.end());
  sort(neg.begin(), neg.end());
  reverse(neg.begin(), neg.end());
  int pcnt = pos.size(), zcnt = zero.size(), ncnt = neg.size();
  LL ALL = 1;
  for (auto &v : a)
    ALL = (ALL * v) % MOD;
  ALL = (ALL + MOD) % MOD;
  if (k == 1) {
    LL ans = *max_element(a.begin(), a.end()) % MOD;
    cout << (ans + MOD) % MOD << '\n';
    return 0;
  }
  if (k == pcnt + zcnt + ncnt) {
    cout << ALL << '\n';
    return 0;
  }
  if (k > pcnt + ncnt) {
    cout << 0 << '\n';
    return 0;
  } else if (pcnt == 0) {
    if (k & 1) {
      if (zcnt) {
        cout << 0 << '\n';
        return 0;
      }
      LL now = 1;
      for (int i = 0; i < k; i++)
        now = (now * neg[i]) % MOD;
      cout << (now + MOD) % MOD << '\n';
      return 0;
    } else {
      if (k > ncnt) {
        cout << 0 << '\n';
        return 0;
      }
      LL now = 1;
      for (int i = ncnt - 1; i >= ncnt - k; i--) {
        now = (now * neg[i]) % MOD;
      }
      cout << (now + MOD) % MOD << '\n';
      return 0;
    }
  } else {
    LL now = 1;
    int i = pos.size() - 1, j = neg.size() - 1;
    for (; k;) {
      if (k == 1) {
        if (i >= 0) {
          if (j >= 1 && i < pcnt - 2 &&
              pos[i] * pos[i + 1] < neg[j] * neg[j - 1]) {
            now = now * qpow(pos[i + 1], MOD - 2, MOD) % MOD;
            now = now * neg[j--] % MOD;
            now = now * neg[j--] % MOD;
          } else
            now = now * pos[i--] % MOD;
        } else if (j >= 1) {
          now = now * qpow(pos[0], MOD - 2, MOD) % MOD;
          now = now * neg[j--] % MOD;
          now = now * neg[j--] % MOD;
        } else {
          if (zcnt)
            now = 0;
          else
            now = now * neg[j] % MOD;
        }
        k--;
      } else {
        LL P = 0, N = 0;
        if (i >= 1)
          P = pos[i] * pos[i - 1];
        else if (i == 0)
          P = pos[i];
        if (j >= 1)
          N = neg[j] * neg[j - 1];
        else if (j == 0)
          N = neg[j];
        if (P > N) {
          now = now * (P % MOD) % MOD;
          k -= 1 + (i > 0);
          i -= 1 + (i > 0);
        } else {
          now = now * (N % MOD) % MOD;
          k -= 1 + (j > 0);
          j -= 1 + (j > 0);
        }
      }
      // trace(k, now);
    }

    cout << (now + MOD) % MOD << '\n';
  }

  return 0;
}