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

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
#define PI 3.14159265359
#define INF 1000100100
#define MOD 1000000007
#define all(x) (x).begin(), (x).end()
typedef long long ll;
#define PP pair<P, P>
using namespace std;

// const int mod = 998244353;
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll 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(ll 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;
  }
};

// 出力は.xで！！！！

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

int main() {
  int n, k;
  cin >> n >> k;
  vector<ll> pos, neg;
  rep(i, n) {
    ll a;
    cin >> a;
    if (a < 0)
      neg.push_back(a);
    else
      pos.push_back(a);
  }
  sort(all(pos));
  sort(all(neg));
  reverse(all(neg));
  int pl = pos.size();
  int nl = neg.size();
  mint ans = 1;
  if (n == k) {
    rep(i, pl) { ans *= pos[i]; }
    rep(i, nl) { ans *= neg[i]; }
    cout << ans.x << endl;
    return 0;
  }
  if (nl == n && k % 2 == 1) {
    rep(i, k) { ans *= neg[i]; }
    cout << ans.x << endl;
    return 0;
  }
  int posp = pl - 1;
  int negp = nl - 1;
  while (k) {
    if (k == 1) {
      ans *= pos[posp];
      break;
    }
    if (posp < 0) {
      ans *= neg[negp--];
      ans *= neg[negp--];
      k -= 2;
      continue;
    }
    if (negp < 1) {
      ans *= pos[posp--];
      k--;
      continue;
    }
    if (posp == 0 && k % 2 == 1) {
      ans *= pos[posp--];
      k--;
      continue;
    }
    if (posp == 0 && k % 2 == 0) {
      ans *= neg[negp--];
      ans *= neg[negp--];
      k -= 2;
    }
    ll can1 = pos[posp] * pos[posp - 1];
    ll can2 = neg[negp] * neg[negp - 1];
    if (can1 > can2) {
      ans *= pos[posp--];
      k--;
    } else {
      ans *= neg[negp--];
      ans *= neg[negp--];
      k -= 2;
    }
  }

  cout << ans.x << endl;

  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
#define PI 3.14159265359
#define INF 1000100100
#define MOD 1000000007
#define all(x) (x).begin(), (x).end()
typedef long long ll;
#define PP pair<P, P>
using namespace std;

// const int mod = 998244353;
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll 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(ll 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;
  }
};

// 出力は.xで！！！！

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

int main() {
  int n, k;
  cin >> n >> k;
  vector<ll> pos, neg;
  rep(i, n) {
    ll a;
    cin >> a;
    if (a < 0)
      neg.push_back(a);
    else
      pos.push_back(a);
  }
  sort(all(pos));
  sort(all(neg));
  reverse(all(neg));
  int pl = pos.size();
  int nl = neg.size();
  mint ans = 1;
  if (n == k) {
    rep(i, pl) { ans *= pos[i]; }
    rep(i, nl) { ans *= neg[i]; }
    cout << ans.x << endl;
    return 0;
  }
  if (nl == n) {
    rep(i, k) { ans *= neg[i]; }
    cout << ans.x << endl;
    return 0;
  }
  int posp = pl - 1;
  int negp = nl - 1;
  while (k) {
    if (k == 1) {
      ans *= pos[posp];
      break;
    }
    if (posp == 0) {
      ans *= neg[negp--];
      ans *= neg[negp--];
      k -= 2;
      continue;
    }
    if (negp < 1) {
      ans *= pos[posp--];
      k--;
      continue;
    }
    if (posp == 1) {
      if (abs(neg[negp - 1]) < pos[posp]) {
        ans *= pos[posp--];
        k--;
      } else {
        ans *= neg[negp--];
        ans *= neg[negp--];
        k -= 2;
        continue;
      }
    }
    ll can1 = pos[posp] * pos[posp - 1];
    ll can2 = neg[negp] * neg[negp - 1];
    if (can1 > can2) {
      ans *= pos[posp--];
      k--;
    } else {
      ans *= neg[negp--];
      ans *= neg[negp--];
      k -= 2;
      continue;
    }
  }

  cout << ans.x << endl;

  return 0;
}