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

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

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define fi first
#define se second
#define debug(...)                                                             \\
  {                                                                            \\
    cerr << "[L" << __LINE__ << "] ";                                          \\
    _debug(__VA_ARGS__);                                                       \\
  }

template <typename T> string join(const vector<T> &v, string del = ", ") {
  stringstream s;
  for (auto x : v)
    s << del << x;
  return s.str().substr(del.size());
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    o << endl;
    rep(i, l) o << (i == 0 ? "[ " : ",\\n  ") << vv[i]
                << (i == l - 1 ? " ]" : "");
  }
  return o;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
inline void _debug() { cerr << endl; }
template <class First, class... Rest>
void _debug(const First &first, const Rest &...rest) {
  cerr << first << " ";
  _debug(rest...);
}

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1 * acos(0.0));
const double EPS = 1e-9;
const int INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3fLL;
const ll mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

int main() {
  ios_base::sync_with_stdio(0);
  // finput("./input");
  int n, k;
  cin >> n >> k;
  vl A(n);
  rep(i, n) cin >> A[i];
  sort(rall(A));

  int l = 0, r = n - 1;
  ll ans = 1;
  if (k & 1)
    ans *= A[0], l = 1;
  rep(i, k / 2) {
    if (A[l] * A[l + 1] >= A[r - 1] * A[r])
      (ans *= A[l] * A[l + 1] % mod) %= mod, l += 2;
    else
      (ans *= A[r] * A[r - 1] % mod) %= mod, r -= 2;
  }
  if (ans < 0) {
    ans = 1;
    rep(i, k)(ans *= mod + A[i]) %= mod;
  }
  cout << ans << endl;
  return 0;
}

Error: 0

Fixed submission:

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

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define fi first
#define se second
#define debug(...)                                                             \\
  {                                                                            \\
    cerr << "[L" << __LINE__ << "] ";                                          \\
    _debug(__VA_ARGS__);                                                       \\
  }

template <typename T> string join(const vector<T> &v, string del = ", ") {
  stringstream s;
  for (auto x : v)
    s << del << x;
  return s.str().substr(del.size());
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    o << endl;
    rep(i, l) o << (i == 0 ? "[ " : ",\\n  ") << vv[i]
                << (i == l - 1 ? " ]" : "");
  }
  return o;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
inline void _debug() { cerr << endl; }
template <class First, class... Rest>
void _debug(const First &first, const Rest &...rest) {
  cerr << first << " ";
  _debug(rest...);
}

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1 * acos(0.0));
const double EPS = 1e-9;
const int INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3fLL;
const ll mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

int main() {
  ios_base::sync_with_stdio(0);
  // finput("./input");
  int n, k;
  cin >> n >> k;
  vl P, M;
  rep(i, n) {
    ll a;
    cin >> a;
    if (a >= 0)
      P.push_back(a);
    else
      M.push_back(-a);
  }
  if (k == n) {
    ll ans = 1;
    rep(i, P.size()) {
      ans *= P[i];
      ans %= mod;
    }
    rep(i, M.size()) {
      ans *= mod - M[i];
      ans %= mod;
    }
    cout << ans << endl;
    return 0;
  }
  if (k & 1 && P.size() == 0) {
    sort(all(M));
    ll ans = 1;
    rep(i, k) {
      ans *= mod - M[i];
      ans %= mod;
    }
    cout << ans << endl;
    return 0;
  }
  P.push_back(1);
  sort(rall(P));
  sort(rall(M));

  int i = 0, j = 0;
  ll ans = 1;
  while (k > i + j) {
    if (i + j == k - 1) {
      ans *= P[i];
      ans %= mod;
      i += 1;
    } else {
      ll p = -INFL, m = -INFL;
      if (i <= P.size() - 1)
        p = P[i];
      if (i + 1 <= P.size() - 1)
        p *= P[i + 1];
      if (i <= M.size() - 1)
        m = M[i];
      if (i + 1 <= M.size() - 1)
        m *= M[i + 1];

      if (i == P.size() - 1 || m >= p) {
        ans *= m;
        ans %= mod;
        j += 2;
      } else {
        ans *= P[i];
        ans %= mod;
        i += 1;
      }
    }
  }
  cout << ans << endl;
  return 0;
}