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

#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ld long double
#define ll long long
#define P pair<int, int>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < n; i++)
#define all(v) v.begin(), v.end()
#define pb push_back
#define eb emplace_back
template <class T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
template <class T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
constexpr int INF = 1000000000000000000;
constexpr int mod = 1000000007;
constexpr double eps = 1e-8;
const double pi = acos(-1);
int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};
int Random(int mi, int ma) {
  random_device rnd;
  mt19937 mt(rnd()); // 32bit
  //[mi,ma]
  uniform_int_distribution<int> engine(mi, ma);
  return engine(mt);
}
int gcd(int a, int b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
int lcm(int a, int b) { return a / gcd(a, b) * b; }
bool prime(int a) {
  if (a == 1)
    return false;
  for (int i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}
int modpow(int a, int b) {
  if (b == 0)
    return 1;
  if (b % 2)
    return modpow(a, b - 1) * a % mod;
  int memo = modpow(a, b / 2);
  return memo * memo % mod;
}
vector<int> kaijo, invkaijo;
void init_fact(int n) {
  kaijo.resize(n + 1);
  invkaijo.resize(n + 1);
  kaijo[0] = 1;
  for (int i = 1; i <= n; i++) {
    kaijo[i] = kaijo[i - 1] * i;
    kaijo[i] %= mod;
  }
  rep(i, n + 1) invkaijo[i] = modpow(kaijo[i], mod - 2);
}
int comb(int a, int b) {
  if (a < b)
    return 0;
  if (a < 0 || b < 0)
    return 0;
  return kaijo[a] * invkaijo[a - b] % mod * invkaijo[b] % mod;
}
int inv(int x) {
  x = modpow(x, mod - 2);
  return x;
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, k, zero = 0;
  cin >> n >> k;
  vector<int> x, y;
  rep(i, n) {
    int a;
    cin >> a;
    if (a > 0)
      x.pb(a);
    if (a < 0)
      y.pb(a);
    if (a == 0)
      zero++;
  }
  sort(all(x));
  sort(all(y));
  int cx = 0, cy = 0;
  while (cy + 1 < y.size() && cy + 2 <= k) {
    cy += 2;
  }
  cx = k - cy;
  if (cx <= x.size()) {
    int pos = x.size() - cx - 1;
    for (int i = cy - 1; i >= 0 && pos - 1 >= 0; i -= 2, pos -= 2) {
      if (x[pos] * x[pos - 1] > y[i] * y[i - 1]) {
        cy -= 2;
      } else
        break;
    }
    int ans = 1;
    for (int i = pos + 1; i < x.size(); i++) {
      ans *= x[i];
      ans %= mod;
    }
    rep(i, cy) {
      ans *= y[i];
      ans %= mod;
    }
    cout << ans << endl;
    return 0;
  }
  if (zero) {
    cout << 0 << endl;
  } else {
    for (int &i : y) {
      i = -i;
      x.pb(i);
    }
    sort(all(x));
    int ans = 1;
    rep(i, k) {
      ans *= x[i];
      ans %= mod;
    }
    cout << mod - ans << endl;
  }
  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ld long double
#define ll long long
#define P pair<int, int>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < n; i++)
#define all(v) v.begin(), v.end()
#define pb push_back
#define eb emplace_back
template <class T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
template <class T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
constexpr int INF = 1000000000000000000;
constexpr int mod = 1000000007;
constexpr double eps = 1e-8;
const double pi = acos(-1);
int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};
int Random(int mi, int ma) {
  random_device rnd;
  mt19937 mt(rnd()); // 32bit
  //[mi,ma]
  uniform_int_distribution<int> engine(mi, ma);
  return engine(mt);
}
int gcd(int a, int b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
int lcm(int a, int b) { return a / gcd(a, b) * b; }
bool prime(int a) {
  if (a == 1)
    return false;
  for (int i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}
int modpow(int a, int b) {
  if (b == 0)
    return 1;
  if (b % 2)
    return modpow(a, b - 1) * a % mod;
  int memo = modpow(a, b / 2);
  return memo * memo % mod;
}
vector<int> kaijo, invkaijo;
void init_fact(int n) {
  kaijo.resize(n + 1);
  invkaijo.resize(n + 1);
  kaijo[0] = 1;
  for (int i = 1; i <= n; i++) {
    kaijo[i] = kaijo[i - 1] * i;
    kaijo[i] %= mod;
  }
  rep(i, n + 1) invkaijo[i] = modpow(kaijo[i], mod - 2);
}
int comb(int a, int b) {
  if (a < b)
    return 0;
  if (a < 0 || b < 0)
    return 0;
  return kaijo[a] * invkaijo[a - b] % mod * invkaijo[b] % mod;
}
int inv(int x) {
  x = modpow(x, mod - 2);
  return x;
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, k, zero = 0;
  cin >> n >> k;
  vector<int> x, y;
  rep(i, n) {
    int a;
    cin >> a;
    if (a > 0)
      x.pb(a);
    if (a < 0)
      y.pb(a);
    if (a == 0)
      zero++;
  }
  sort(all(x));
  sort(all(y));
  if (k <= x.size() || (k % 2 == 0 && y.size() >= k) ||
      (k % 2 == 0 && k - y.size() / 2 * 2 <= x.size()) ||
      (k % 2 == 1 && k - y.size() / 2 * 2 <= x.size())) {
    int cx = 0, cy = 0;
    if (k <= x.size())
      cx = k, cy = 0;
    else {
      if ((k - x.size()) % 2 == 0)
        cx = x.size(), cy = k - x.size();
      else
        cx = x.size() - 1, cy = k - x.size() + 1;
    }
    int pos = 0;
    while (pos + 1 < x.size() && pos + 1 < y.size()) {
      if (x[pos] * x[pos + 1] < y[pos] * y[pos + 1]) {
        cx -= 2, cy += 2;
        pos += 2;
      } else
        break;
    }
    reverse(all(x));
    int ans = 1;
    rep(i, cx) {
      ans *= x[i];
      ans %= mod;
    }
    rep(i, cy) {
      ans *= y[i];
      ans %= mod;
    }
    cout << ans << endl;
  } else if (zero) {
    cout << 0 << endl;
  } else {
    for (int &i : y) {
      i = -i;
      x.pb(i);
    }
    sort(all(x));
    int ans = 1;
    rep(i, k) {
      ans *= x[i];
      ans %= mod;
    }
    cout << mod - ans << endl;
  }
  return 0;
}