Problem p02965 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
const ll INF = 1LL << 60;
const ll MOD = 998244353;
template <class T> bool chmax(T &a, const T &b) {
  return (a < b) ? (a = b, 1) : 0;
}
template <class T> bool chmin(T &a, const T &b) {
  return (b < a) ? (a = b, 1) : 0;
}
template <class C> void print(const C &c, std::ostream &os = std::cout) {
  std::copy(std::begin(c), std::end(c),
            std::ostream_iterator<typename C::value_type>(os, " "));
  os << std::endl;
}

// mod int struct
// original : https://github.com/beet-aizu/library/blob/master/mod/mint.cpp

struct mint {
  ll v;
  ll mod;
  mint() : v(0) {}
  mint(signed v, ll mod = MOD) : v(v), mod(mod) {}
  mint(ll t, ll mod = MOD) : mod(mod) {
    v = t % mod;
    if (v < 0)
      v += mod;
  }

  mint pow(ll k) {
    mint res(1), tmp(v);
    while (k) {
      if (k & 1)
        res *= tmp;
      tmp *= tmp;
      k >>= 1;
    }
    return res;
  }

  static mint add_identity() { return mint(0); }
  static mint mul_identity() { return mint(1); }

  mint inv() { return pow(mod - 2); }

  mint &operator+=(mint a) {
    v += a.v;
    if (v >= mod)
      v -= mod;
    return *this;
  }
  mint &operator-=(mint a) {
    v += mod - a.v;
    if (v >= mod)
      v -= mod;
    return *this;
  }
  mint &operator*=(mint a) {
    v = 1LL * v * a.v % mod;
    return *this;
  }
  mint &operator/=(mint a) { return (*this) *= a.inv(); }

  mint operator+(mint a) const { return mint(v) += a; };
  mint operator-(mint a) const { return mint(v) -= a; };
  mint operator*(mint a) const { return mint(v) *= a; };
  mint operator/(mint a) const { return mint(v) /= a; };

  mint operator-() const { return v ? mint(mod - v) : mint(v); }

  bool operator==(const mint a) const { return v == a.v; }
  bool operator!=(const mint a) const { return v != a.v; }
  bool operator<(const mint a) const { return v < a.v; }

  // find x s.t. a^x = b
  static ll log(ll a, ll b) {
    const ll sq = 40000;
    unordered_map<ll, ll> dp;
    dp.reserve(sq);
    mint res(1);
    for (int r = 0; r < sq; r++) {
      if (!dp.count(res.v))
        dp[res.v] = r;
      res *= a;
    }
    mint p = mint(a).inv().pow(sq);
    res = b;
    for (int q = 0; q <= MOD / sq + 1; q++) {
      if (dp.count(res.v)) {
        ll idx = q * sq + dp[res.v];
        if (idx > 0)
          return idx;
      }
      res *= p;
    }
    assert(0);
    return ll(-1);
  }

  static mint comb(long long n, int k) {
    mint num(1), dom(1);
    for (int i = 0; i < k; i++) {
      num *= mint(n - i);
      dom *= mint(i + 1);
    }
    return num / dom;
  }
};
ostream &operator<<(ostream &os, mint m) {
  os << m.v;
  return os;
}

struct Combination {
  vector<ll> fac, finv, inv;

  Combination(ll maxN) {
    maxN += 100; // for safety
    fac.resize(maxN + 1);
    finv.resize(maxN + 1);
    inv.resize(maxN + 1);
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i <= maxN; ++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;
    }
  }
  ll operator()(ll n, ll k) {
    if (n < k)
      return 0;
    if (n < 0 || k < 0)
      return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
  }
};

int main() {
  int n, m;
  cin >> n >> m;
  // oddになる数で場合分け出来そう
  // m < n
  Combination nCk(1000000);
  mint ret = 0;
  for (int i = m % 2; i <= m; i += 2) { // i個oddに使う
    ll mm = (3 * m - i) / 2;
    ret += mint(nCk(n, i)) * mint(nCk(mm + n - 1, n - 1));
  }
  for (int i = m % 2; i <= m; i += 2) {
    ll mm = (m - i) / 2;
    mint temp = mint(nCk(n, i)) * mint(nCk(mm + n - 1, n - 1));
    temp -= mint(nCk(n - 1, i)) * mint(nCk(mm + n - 2, n - 2));
    ret -= temp * n;
  }
  cout << ret << "\n";
  return 0;
}

Fixed submission:
#include "bits/stdc++.h"
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
const ll INF = 1LL << 60;
const ll MOD = 998244353;
template <class T> bool chmax(T &a, const T &b) {
  return (a < b) ? (a = b, 1) : 0;
}
template <class T> bool chmin(T &a, const T &b) {
  return (b < a) ? (a = b, 1) : 0;
}
template <class C> void print(const C &c, std::ostream &os = std::cout) {
  std::copy(std::begin(c), std::end(c),
            std::ostream_iterator<typename C::value_type>(os, " "));
  os << std::endl;
}

// mod int struct
// original : https://github.com/beet-aizu/library/blob/master/mod/mint.cpp

struct mint {
  ll v;
  ll mod;
  mint() : v(0) {}
  mint(signed v, ll mod = MOD) : v(v), mod(mod) {}
  mint(ll t, ll mod = MOD) : mod(mod) {
    v = t % mod;
    if (v < 0)
      v += mod;
  }

  mint pow(ll k) {
    mint res(1), tmp(v);
    while (k) {
      if (k & 1)
        res *= tmp;
      tmp *= tmp;
      k >>= 1;
    }
    return res;
  }

  static mint add_identity() { return mint(0); }
  static mint mul_identity() { return mint(1); }

  mint inv() { return pow(mod - 2); }

  mint &operator+=(mint a) {
    v += a.v;
    if (v >= mod)
      v -= mod;
    return *this;
  }
  mint &operator-=(mint a) {
    v += mod - a.v;
    if (v >= mod)
      v -= mod;
    return *this;
  }
  mint &operator*=(mint a) {
    v = 1LL * v * a.v % mod;
    return *this;
  }
  mint &operator/=(mint a) { return (*this) *= a.inv(); }

  mint operator+(mint a) const { return mint(v) += a; };
  mint operator-(mint a) const { return mint(v) -= a; };
  mint operator*(mint a) const { return mint(v) *= a; };
  mint operator/(mint a) const { return mint(v) /= a; };

  mint operator-() const { return v ? mint(mod - v) : mint(v); }

  bool operator==(const mint a) const { return v == a.v; }
  bool operator!=(const mint a) const { return v != a.v; }
  bool operator<(const mint a) const { return v < a.v; }

  // find x s.t. a^x = b
  static ll log(ll a, ll b) {
    const ll sq = 40000;
    unordered_map<ll, ll> dp;
    dp.reserve(sq);
    mint res(1);
    for (int r = 0; r < sq; r++) {
      if (!dp.count(res.v))
        dp[res.v] = r;
      res *= a;
    }
    mint p = mint(a).inv().pow(sq);
    res = b;
    for (int q = 0; q <= MOD / sq + 1; q++) {
      if (dp.count(res.v)) {
        ll idx = q * sq + dp[res.v];
        if (idx > 0)
          return idx;
      }
      res *= p;
    }
    assert(0);
    return ll(-1);
  }

  static mint comb(long long n, int k) {
    mint num(1), dom(1);
    for (int i = 0; i < k; i++) {
      num *= mint(n - i);
      dom *= mint(i + 1);
    }
    return num / dom;
  }
};
ostream &operator<<(ostream &os, mint m) {
  os << m.v;
  return os;
}

struct Combination {
  vector<ll> fac, finv, inv;

  Combination(ll maxN) {
    maxN += 100; // for safety
    fac.resize(maxN + 1);
    finv.resize(maxN + 1);
    inv.resize(maxN + 1);
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i <= maxN; ++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;
    }
  }
  ll operator()(ll n, ll k) {
    if (n < k)
      return 0;
    if (n < 0 || k < 0)
      return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
  }
};

int main() {
  int n, m;
  cin >> n >> m;
  // oddになる数で場合分け出来そう
  // m < n
  Combination nCk(10000000);
  mint ret = 0;
  for (int i = m % 2; i <= m; i += 2) { // i個oddに使う
    ll mm = (3 * m - i) / 2;
    ret += mint(nCk(n, i)) * mint(nCk(mm + n - 1, n - 1));
  }
  for (int i = m % 2; i <= m; i += 2) {
    ll mm = (m - i) / 2;
    mint temp = mint(nCk(n, i)) * mint(nCk(mm + n - 1, n - 1));
    temp -= mint(nCk(n - 1, i)) * mint(nCk(mm + n - 2, n - 2));
    ret -= temp * n;
  }
  cout << ret << "\n";
  return 0;
}