Problem p02616 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, N) for (int i = 0; i < int(N); ++i)
#define rep1(i, N) for (int i = 1; i < int(N); ++i)
#define all(a) (a).begin(), (a).end()
#define print(v)                                                               \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << _ << ", ";                                                       \
    cerr << "]" << endl;                                                       \
  }
#define printpair(v)                                                           \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << "{" << _.first << "," << _.second << "}"                         \
           << ", ";                                                            \
    cerr << "]" << endl;                                                       \
  }
#define dump(x) cerr << #x << ": " << x << endl;
#define bit(k) (1LL << (k))
typedef long long ll;
typedef pair<int, int> i_i;
typedef pair<ll, ll> l_l;
template <class T> using vec = vector<T>;
template <class T> using vvec = vector<vec<T>>;
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "{" << p.first << ", " << p.second << "}";
  return os;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

const int INF = (ll)1e9;
const ll INFLL = (ll)1e18 + 1;
const ll MOD = (ll)1e9 + 7;
const double PI = acos(-1.0);
/*
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
const string dir = "DRUL";
*/

struct mint {
  long long x;
  mint(long long _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 += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint modpow(long long t) const {
    if (!t)
      return 1;
    mint a = modpow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return modpow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
  friend std::ostream &operator<<(std::ostream &os, const mint &a) {
    os << a.x;
    return os;
  }
};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  ll N, K;
  cin >> N >> K;
  vector<ll> s, f;
  vector<ll> A;
  ll cntz = 0, cntpl = 0, cntmi = 0;
  rep(i, N) {
    ll a;
    cin >> a;
    A.push_back(abs(a));
    if (a == 0)
      cntz++;
    if (a > 0) {
      s.push_back(a);
      cntpl++;
    }
    if (a < 0) {
      f.push_back(abs(a));
      cntmi++;
    }
  }
  sort(all(A));
  sort(all(s), greater<>());
  sort(all(f), greater<>());
  mint ans = 1;
  if (N == K) {
    rep(i, K) ans *= A[i];
    if (cntmi & 1)
      ans = -ans;
    cout << ans << endl;
    return 0;
  }
  // K < N;
  ll idx[] = {0, 0};
  if (K & 1) {
    if (cntpl > 0) {
      ans *= s[0];
      idx[0]++;
    }
    //
    else {
      if (cntz > 0)
        cout << 0 << endl;
      else {
        rep(i, K) { ans *= A[i]; }
        ans = -ans;
        cout << ans << endl;
      }
      return 0;
    }
  }
  // 残りの偶数個を決めていく
  rep(_, K / 2) {
    ll tmp[2];
    tmp[0] = s[idx[0]] * s[idx[0] + 1];
    tmp[1] = f[idx[1]] * f[idx[1] + 1];
    if (tmp[0] < tmp[1]) {
      ans *= tmp[1];
      idx[1] += 2;
    } else {
      ans *= tmp[0];
      idx[0] += 2;
    }
  }
  cout << ans << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, N) for (int i = 0; i < int(N); ++i)
#define rep1(i, N) for (int i = 1; i < int(N); ++i)
#define all(a) (a).begin(), (a).end()
#define print(v)                                                               \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << _ << ", ";                                                       \
    cerr << "]" << endl;                                                       \
  }
#define printpair(v)                                                           \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << "{" << _.first << "," << _.second << "}"                         \
           << ", ";                                                            \
    cerr << "]" << endl;                                                       \
  }
#define dump(x) cerr << #x << ": " << x << endl;
#define bit(k) (1LL << (k))
typedef long long ll;
typedef pair<int, int> i_i;
typedef pair<ll, ll> l_l;
template <class T> using vec = vector<T>;
template <class T> using vvec = vector<vec<T>>;
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "{" << p.first << ", " << p.second << "}";
  return os;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

const int INF = (ll)1e9;
const ll INFLL = (ll)1e18 + 1;
const ll MOD = (ll)1e9 + 7;
const double PI = acos(-1.0);
/*
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
const string dir = "DRUL";
*/

struct mint {
  long long x;
  mint(long long _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 += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint modpow(long long t) const {
    if (!t)
      return 1;
    mint a = modpow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return modpow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
  friend std::ostream &operator<<(std::ostream &os, const mint &a) {
    os << a.x;
    return os;
  }
};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  ll N, K;
  cin >> N >> K;
  vector<ll> s(K, 0), f(K, 0);
  vector<ll> A;
  ll cntz = 0, cntpl = 0, cntmi = 0;
  rep(i, N) {
    ll a;
    cin >> a;
    A.push_back(abs(a));
    if (a == 0)
      cntz++;
    if (a > 0) {
      s.push_back(a);
      cntpl++;
    }
    if (a < 0) {
      f.push_back(abs(a));
      cntmi++;
    }
  }
  sort(all(A));
  sort(all(s), greater<>());
  sort(all(f), greater<>());
  mint ans = 1;
  if (N == K) {
    rep(i, K) ans *= A[i];
    if (cntmi & 1)
      ans = -ans;
    cout << ans << endl;
    return 0;
  }
  // K < N;
  ll idx[] = {0, 0};
  if (K & 1) {
    if (cntpl > 0) {
      ans *= s[0];
      idx[0]++;
    }
    //
    else {
      if (cntz > 0)
        cout << 0 << endl;
      else {
        rep(i, K) { ans *= A[i]; }
        ans = -ans;
        cout << ans << endl;
      }
      return 0;
    }
  }
  // 残りの偶数個を決めていく
  rep(_, K / 2) {
    ll tmp[2];
    tmp[0] = s[idx[0]] * s[idx[0] + 1];
    tmp[1] = f[idx[1]] * f[idx[1] + 1];
    if (tmp[0] < tmp[1]) {
      ans *= tmp[1];
      idx[1] += 2;
    } else {
      ans *= tmp[0];
      idx[0] += 2;
    }
  }
  cout << ans << endl;
}