Problem p02616 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include <boost/multiprecision/cpp_int.hpp>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
#pragma GCC optimize("O3")
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPP(i, n) for (int i = 1; i <= n; i++)
#define ALL(obj) (obj).begin(), (obj).end()
#define EPS (1e-9)
#define INF (1e17)
#define PI (acos(-1))
// const double PI = acos(-1);
// const double EPS = 1e-15;
// long long INF=(long long)1E17;
#define i_7 (long long)(1e9 + 7)
// #define i_7 998'244'353
long mod(long a) {
  long long c = a % i_7;
  if (c >= 0)
    return c;
  return c + i_7;
}
long long po(long a, long b) {
  if (b == 0) {
    return 1;
  }

  long long z = po(a, b / 2);
  z = mod(z * z);
  if (b % 2 != 0) {
    z = mod(a * z);
  }
  return z;
}

bool prime_(int n) {
  if (n == 1) {
    return false;
  } else if (n == 2) {
    return true;
  } else {
    for (int i = 2; i <= std::sqrt(n); i++) {
      if (n % i == 0) {
        return false;
      }
    }
    return true;
  }
}

long long gcd_(long long a, long long b) {
  if (a < b) {
    std::swap(a, b);
  }
  if (a % b == 0) {
    return b;
  } else {
    return gcd_(b, a % b);
  }
}

long long lcm_(long long x, long long y) { return (x / gcd_(x, y)) * y; }

// using namespace std;
// using namespace boost::multiprecision;
// using namespace __gnu_pbds;

int main() {
  using namespace std;
  int n, k;
  cin >> n >> k;
  vector<long long> a(n);
  bool all_minus = true;
  REP(i, n) {
    cin >> a[i];
    if (a[i] >= 0)
      all_minus = false;
  }
  auto cmp = [](long long x, long long y) -> bool { return abs(x) >= abs(y); };
  sort(ALL(a), cmp);
  long long ans = 1;
  if (k == n) {
    REP(i, k) {
      ans *= a[i];
      ans = mod(ans);
    }
    cout << ans << endl;
    return 0;
  }
  if (all_minus && k % 2 == 1) {
    reverse(ALL(a));
    REP(i, k) {
      ans *= a[i];
      ans = mod(ans);
    }
    cout << ans << endl;
    return 0;
  }

  queue<long long> s;
  deque<long long> pos;
  deque<long long> neg;
  REP(i, n) {
    if (a[i] >= 0LL) {
      pos.push_back(a[i]);
    } else {
      neg.push_back(a[i]);
    }
  }
  while ((int)s.size() < k) {
    if ((int)s.size() == (k - 1)) {
      long long p1 = pos.front();
      pos.pop_front();
      s.push(p1);
      break;
    }
    if ((int)neg.size() < 2) {
      long long p1 = pos.front();
      pos.pop_front();
      s.push(p1);
    } else if (pos.empty()) {
      long long n1 = neg.front();
      neg.pop_front();
      long long n2 = neg.front();
      neg.pop_front();
      s.push(n1);
      s.push(n2);
    } else if ((int)pos.size() < 2) {
      long long p1 = pos.front();
      pos.pop_front();
      long long n1 = neg.front();
      neg.pop_front();
      long long n2 = neg.front();
      neg.pop_front();
      if (p1 >= n1 * n2) {
        s.push(p1);
        neg.push_front(n2);
        neg.push_front(n1);
      } else {
        s.push(n1);
        s.push(n2);
        pos.push_front(p1);
      }
    } else {
      long long p1 = pos.front();
      pos.pop_front();
      long long p2 = pos.front();
      pos.pop_front();
      long long n1 = neg.front();
      neg.pop_front();
      long long n2 = neg.front();
      neg.pop_front();
      if (p1 * p2 >= n1 * n2) {
        s.push(p1);
        pos.push_front(p2);
        neg.push_front(n2);
        neg.push_front(n1);
      } else {
        s.push(n1);
        s.push(n2);
        pos.push_front(p2);
        pos.push_front(p1);
      }
    }
  }

  while (!s.empty()) {
    long long x = s.front();
    s.pop();
    ans *= x;
    ans = mod(ans);
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
// #include <boost/multiprecision/cpp_int.hpp>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
#pragma GCC optimize("O3")
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPP(i, n) for (int i = 1; i <= n; i++)
#define ALL(obj) (obj).begin(), (obj).end()
#define EPS (1e-9)
#define INF (1e17)
#define PI (acos(-1))
// const double PI = acos(-1);
// const double EPS = 1e-15;
// long long INF=(long long)1E17;
#define i_7 (long long)(1e9 + 7)
// #define i_7 998'244'353
long mod(long a) {
  long long c = a % i_7;
  if (c >= 0)
    return c;
  return c + i_7;
}
long long po(long a, long b) {
  if (b == 0) {
    return 1;
  }

  long long z = po(a, b / 2);
  z = mod(z * z);
  if (b % 2 != 0) {
    z = mod(a * z);
  }
  return z;
}

bool prime_(int n) {
  if (n == 1) {
    return false;
  } else if (n == 2) {
    return true;
  } else {
    for (int i = 2; i <= std::sqrt(n); i++) {
      if (n % i == 0) {
        return false;
      }
    }
    return true;
  }
}

long long gcd_(long long a, long long b) {
  if (a < b) {
    std::swap(a, b);
  }
  if (a % b == 0) {
    return b;
  } else {
    return gcd_(b, a % b);
  }
}

long long lcm_(long long x, long long y) { return (x / gcd_(x, y)) * y; }

// using namespace std;
// using namespace boost::multiprecision;
// using namespace __gnu_pbds;

int main() {
  using namespace std;
  int n, k;
  cin >> n >> k;
  vector<long long> a(n);
  bool all_minus = true;
  REP(i, n) {
    cin >> a[i];
    if (a[i] >= 0)
      all_minus = false;
  }
  auto cmp = [](long long x, long long y) -> bool { return abs(x) > abs(y); };
  sort(ALL(a), cmp);
  long long ans = 1;
  if (k == n) {
    REP(i, k) {
      ans *= a[i];
      ans = mod(ans);
    }
    cout << ans << endl;
    return 0;
  }
  if (all_minus && k % 2 == 1) {
    reverse(ALL(a));
    REP(i, k) {
      ans *= a[i];
      ans = mod(ans);
    }
    cout << ans << endl;
    return 0;
  }

  queue<long long> s;
  deque<long long> pos;
  deque<long long> neg;
  REP(i, n) {
    if (a[i] >= 0LL) {
      pos.push_back(a[i]);
    } else {
      neg.push_back(a[i]);
    }
  }
  while ((int)s.size() < k) {
    if ((int)s.size() == (k - 1)) {
      long long p1 = pos.front();
      pos.pop_front();
      s.push(p1);
      break;
    }
    if ((int)neg.size() < 2) {
      long long p1 = pos.front();
      pos.pop_front();
      s.push(p1);
    } else if (pos.empty()) {
      long long n1 = neg.front();
      neg.pop_front();
      long long n2 = neg.front();
      neg.pop_front();
      s.push(n1);
      s.push(n2);
    } else if ((int)pos.size() < 2) {
      long long p1 = pos.front();
      pos.pop_front();
      long long n1 = neg.front();
      neg.pop_front();
      long long n2 = neg.front();
      neg.pop_front();
      if (p1 >= n1 * n2) {
        s.push(p1);
        neg.push_front(n2);
        neg.push_front(n1);
      } else {
        s.push(n1);
        s.push(n2);
        pos.push_front(p1);
      }
    } else {
      long long p1 = pos.front();
      pos.pop_front();
      long long p2 = pos.front();
      pos.pop_front();
      long long n1 = neg.front();
      neg.pop_front();
      long long n2 = neg.front();
      neg.pop_front();
      if (p1 * p2 >= n1 * n2) {
        s.push(p1);
        pos.push_front(p2);
        neg.push_front(n2);
        neg.push_front(n1);
      } else {
        s.push(n1);
        s.push(n2);
        pos.push_front(p2);
        pos.push_front(p1);
      }
    }
  }

  while (!s.empty()) {
    long long x = s.front();
    s.pop();
    ans *= x;
    ans = mod(ans);
  }
  cout << ans << endl;
  return 0;
}