Problem p02965 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>
#define MOD 998244353
#define mkp make_pair
typedef long long ll;
using namespace std;

#define MAX_N 200020
ll inv[MAX_N + 10], fac[MAX_N + 10], ifac[MAX_N + 10];

void setComb() {
  inv[0] = 1;
  inv[1] = 1;
  fac[1] = 1;
  ifac[1] = 1;
  fac[0] = 1;
  ifac[0] = 1;
  for (int i = 2; i < MAX_N; i++) {
    inv[i] = (-MOD / i) * inv[MOD % i] % MOD;
    fac[i] = fac[i - 1] * i % MOD;
    ifac[i] = ifac[i - 1] * inv[i] % MOD;

    inv[i] = (inv[i] + MOD) % MOD;
    fac[i] = (fac[i] + MOD) % MOD;
    ifac[i] = (ifac[i] + MOD) % MOD;
  }
  return;
}

ll comb(ll n, ll k) {
  if (n < k || n < 0 || k < 0)
    return 0;
  else
    return ((fac[n] * ifac[k] % MOD * ifac[n - k] % MOD + MOD) % MOD);
}

ll hcomb(ll n, ll r) {
  if (n == 0 && r == 0)
    return 1;
  else if (n < 0 || r < 0)
    return 0;
  else
    return comb(n + r - 1, r);
}

ll mod_pow(ll x, ll n) {
  x %= MOD;
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * x % MOD;
    x = x * x % MOD;
    n >>= 1;
  }
  return res;
}

void add(ll &a, ll b) { a = (a + b) % MOD; }

void mul(ll &a, ll b) {
  a %= MOD;
  b %= MOD;
  a = a * b % MOD;
}

int main() {
  ll N, M;
  cin >> N >> M;

  setComb();

  ll nans = 0;
  for (ll k = 0; k < M; k++) {
    ll res = hcomb(N - 1, k);
    mul(res, N);
    add(nans, res);
  }

  ll ans = 0;
  for (ll k = 0; k <= min(N, M); k++) {
    if (k % 2 != M % 2)
      continue;
    ll res = hcomb(N, (3 * M - k) / 2);
    mul(res, comb(N, k));
    add(ans, res);
  }

  add(ans, -nans + MOD);
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>
#define MOD 998244353
#define mkp make_pair
typedef long long ll;
using namespace std;

#define MAX_N 4000040
ll inv[MAX_N + 10], fac[MAX_N + 10], ifac[MAX_N + 10];

void setComb() {
  inv[0] = 1;
  inv[1] = 1;
  fac[1] = 1;
  ifac[1] = 1;
  fac[0] = 1;
  ifac[0] = 1;
  for (int i = 2; i < MAX_N; i++) {
    inv[i] = (-MOD / i) * inv[MOD % i] % MOD;
    fac[i] = fac[i - 1] * i % MOD;
    ifac[i] = ifac[i - 1] * inv[i] % MOD;

    inv[i] = (inv[i] + MOD) % MOD;
    fac[i] = (fac[i] + MOD) % MOD;
    ifac[i] = (ifac[i] + MOD) % MOD;
  }
  return;
}

ll comb(ll n, ll k) {
  if (n < k || n < 0 || k < 0)
    return 0;
  else
    return ((fac[n] * ifac[k] % MOD * ifac[n - k] % MOD + MOD) % MOD);
}

ll hcomb(ll n, ll r) {
  if (n == 0 && r == 0)
    return 1;
  else if (n < 0 || r < 0)
    return 0;
  else
    return comb(n + r - 1, r);
}

ll mod_pow(ll x, ll n) {
  x %= MOD;
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * x % MOD;
    x = x * x % MOD;
    n >>= 1;
  }
  return res;
}

void add(ll &a, ll b) { a = (a + b) % MOD; }

void mul(ll &a, ll b) {
  a %= MOD;
  b %= MOD;
  a = a * b % MOD;
}

int main() {
  ll N, M;
  cin >> N >> M;

  setComb();

  ll nans = 0;
  for (ll k = 0; k < M; k++) {
    ll res = hcomb(N - 1, k);
    mul(res, N);
    add(nans, res);
  }

  ll ans = 0;
  for (ll k = 0; k <= min(N, M); k++) {
    if (k % 2 != M % 2)
      continue;
    ll res = hcomb(N, (3 * M - k) / 2);
    mul(res, comb(N, k));
    add(ans, res);
  }

  add(ans, -nans + MOD);
  cout << ans << endl;

  return 0;
}