Problem p02616 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define int ll
#define ll long long
#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007
// #define MOD 998244353
#define MEM_SIZE 101010
#define DEBUG_OUT true
#define ALL(x) (x).begin(), (x).end()

template <typename T> void DEBUG(T e) {
  if (DEBUG_OUT == false)
    return;
  std::cout << e << " ";
}
template <typename T> void DEBUG(const std::vector<T> &v) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &e : v) {
    std::cout << e << " ";
  }
  std::cout << std::endl;
}
template <typename T> void DEBUG(const std::vector<std::vector<T>> &vv) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &v : vv) {
    DEBUG(v);
  }
}
template <class T, class... Ts> void DEBUG(T d, Ts... e) {
  if (DEBUG_OUT == false)
    return;
  DEBUG(d);
  DEBUG(e...);
}
template <class T> void corner(bool flg, T hoge) {
  if (flg) {
    cout << hoge << endl;
    abort();
  }
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

// BELOW

// BELOW

struct mint {
  int x; // typedef long long int;
  mint(int x = 0) : x((x % MOD + MOD) % MOD) {}
  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 pow(int t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

const int MAX = 510000;

long long fac[MAX], finv[MAX], inv[MAX];
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

int COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int PERM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[n - k] % MOD) % MOD;
}

int MCOM(int n, int k) {
  // n個のものから重複を許してr個選ぶ。
  // Choose R from N types of things, allowing for duplication.
  return COM(n + k - 1, k);
}
// solve

//-----CODE------//

// https://atcoder.jp/contests/agc036/submissions/6479142
void solve(void) {
  int n, k;
  cin >> n >> k;
  vector<pair<int, int>> vp;
  int ma = -1 * INF;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    vp.emplace_back(abs(x), x);
    chmax(ma, x);
  }
  sort(ALL(vp));
  reverse(ALL(vp));
  int cnt_minus = 0;
  int cnt_zero = 0;
  mint res = 1;
  if (k == 1) {
    if (ma < 0)
      ma = MOD + ma;
    cout << ma << endl;
    return;
  }

  if (ma < 0 and k % 2 == 1) {
    reverse(ALL(vp));
    for (int i = 0; i < k; i++) {
      res *= vp[i].first;
    }

    cout << MOD - res.x << endl;
    return;
  }

  if (k % 2 == 1 and ma == 0 || vp[k - 1].first == 0) {
    cout << 0 << endl;
    return;
  }
  for (int i = 0; i < k; i++) {
    res *= vp[i].first;
    if (vp[i].second < 0)
      cnt_minus += 1;
    else if (vp[i].second == 0)
      cnt_zero += 1;
  }

  if (n == k || cnt_minus % 2 == 0) {
    cout << res.x << endl;
    return;
  }
  abort();
  // abort();
  int f1 = 0, f2 = 0;
  for (int i = k; i < n; i++) {
    if (vp[i].second > 0) {
      f1 = vp[i].first;
      break;
    }
  }

  for (int i = k; i < n; i++) {
    if (vp[i].second < 0) {
      f2 = vp[i].first;
      break;
    }
  }
  int g1 = 0, g2 = 0;
  for (int i = k - 1; i >= 0; i--) {
    if (vp[i].second < 0) {
      g1 = vp[i].first;
      break;
    }
  }

  for (int i = k - 1; i >= 0; i--) {
    if (vp[i].second > 0) {
      g2 = vp[i].first;
      break;
    }
  }

  if (abs(f1 * g2) > abs(f2 * g1) || g2 == 0) {
    res *= f1;
    res /= g1;
    cout << res.x << endl;
  } else {
    res *= f2;
    res /= g2;
    cout << res.x << endl;
  }

  return;
}
int32_t main(int32_t argc, const char *argv[]) {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(11);
  solve();

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define int ll
#define ll long long
#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007
// #define MOD 998244353
#define MEM_SIZE 101010
#define DEBUG_OUT true
#define ALL(x) (x).begin(), (x).end()

template <typename T> void DEBUG(T e) {
  if (DEBUG_OUT == false)
    return;
  std::cout << e << " ";
}
template <typename T> void DEBUG(const std::vector<T> &v) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &e : v) {
    std::cout << e << " ";
  }
  std::cout << std::endl;
}
template <typename T> void DEBUG(const std::vector<std::vector<T>> &vv) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &v : vv) {
    DEBUG(v);
  }
}
template <class T, class... Ts> void DEBUG(T d, Ts... e) {
  if (DEBUG_OUT == false)
    return;
  DEBUG(d);
  DEBUG(e...);
}
template <class T> void corner(bool flg, T hoge) {
  if (flg) {
    cout << hoge << endl;
    abort();
  }
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

// BELOW

// BELOW

struct mint {
  int x; // typedef long long int;
  mint(int x = 0) : x((x % MOD + MOD) % MOD) {}
  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 pow(int t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

const int MAX = 510000;

long long fac[MAX], finv[MAX], inv[MAX];
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

int COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int PERM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[n - k] % MOD) % MOD;
}

int MCOM(int n, int k) {
  // n個のものから重複を許してr個選ぶ。
  // Choose R from N types of things, allowing for duplication.
  return COM(n + k - 1, k);
}
// solve

//-----CODE------//

// https://atcoder.jp/contests/agc036/submissions/6479142
void solve(void) {
  int n, k;
  cin >> n >> k;
  vector<pair<int, int>> vp;
  int ma = -1 * INF;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    vp.emplace_back(abs(x), x);
    chmax(ma, x);
  }
  sort(ALL(vp));
  reverse(ALL(vp));
  int cnt_minus = 0;
  int cnt_zero = 0;
  mint res = 1;
  if (k == 1) {
    if (ma < 0)
      ma = MOD + ma;
    cout << ma << endl;
    return;
  }

  if (ma < 0 and k % 2 == 1) {
    reverse(ALL(vp));
    for (int i = 0; i < k; i++) {
      res *= vp[i].first;
    }

    cout << MOD - res.x << endl;
    return;
  }

  if (k % 2 == 1 and ma == 0 || vp[k - 1].first == 0) {
    cout << 0 << endl;
    return;
  }
  for (int i = 0; i < k; i++) {
    res *= vp[i].first;
    if (vp[i].second < 0)
      cnt_minus += 1;
    else if (vp[i].second == 0)
      cnt_zero += 1;
  }

  if (n == k || cnt_minus % 2 == 0) {
    if (cnt_minus % 2 == 0)
      cout << res.x << endl;
    else {
      cout << MOD - res.x << endl;
    }

    return;
  }
  // abort();
  // abort();
  int f1 = 0, f2 = 0;
  for (int i = k; i < n; i++) {
    if (vp[i].second > 0) {
      f1 = vp[i].first;
      break;
    }
  }

  for (int i = k; i < n; i++) {
    if (vp[i].second < 0) {
      f2 = vp[i].first;
      break;
    }
  }
  int g1 = 0, g2 = 0;
  for (int i = k - 1; i >= 0; i--) {
    if (vp[i].second < 0) {
      g1 = vp[i].first;
      break;
    }
  }

  for (int i = k - 1; i >= 0; i--) {
    if (vp[i].second > 0) {
      g2 = vp[i].first;
      break;
    }
  }

  if (abs(f1 * g2) > abs(f2 * g1) || g2 == 0) {
    res *= f1;
    res /= g1;
    cout << res.x << endl;
  } else {
    res *= f2;
    res /= g2;
    cout << res.x << endl;
  }

  return;
}
int32_t main(int32_t argc, const char *argv[]) {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(11);
  solve();

  return 0;
}