Problem p03332 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;
#define _USE_MATH_DEFINES_
#define ll long long
#define ld long double
#define Accepted 0
#define pb push_back
#define mp make_pair
#define sz(x) (int)(x.size())
#define every(x) x.begin(), x.end()
#define F first
#define S second
#define lb lower_bound
#define ub upper_bound
#define For(i, x, y) for (ll i = x; i <= y; i++)
#define FOr(i, x, y) for (ll i = x; i >= y; i--)
#define SpeedForce ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)
// ROAD to... Red

inline void Input_Output() {
  // freopen(".in", "r", stdin);
  // freopen(".out", "w", stdout);
}

const double eps = 0.000001;
const ld pi = acos(-1);
const int maxn = 1e7 + 9;
const int mod = 998244353;
const ll MOD = 1e18 + 9;
const ll INF = 1e18 + 123;
const int inf = 2e9 + 11;
const int mxn = 1e6 + 9;
const int N = 6e5 + 123;
const int M = 22;
const int pri = 997;
const int Magic = 2101;

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};

ll n, a, b, k;
ll f[N];
ll rn[N];
ll inv[N];
ll left[N];

ll C(int n, int k) {
  if (k > n)
    return 0;
  return (f[n] * inv[k] % mod) * inv[n - k] % mod;
}

ll bpow(ll a, ll b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1)
      res = (res * a) % mod;
    a = (a * a) % mod;
    b /= 2;
  }
  return res;
}

int main() {
  SpeedForce;

  cin >> n >> a >> b >> k;

  f[0] = 1;
  rn[1] = 1;
  inv[0] = 1;
  f[1] = inv[1] = 1;
  for (int i = 2; i <= n; ++i)
    rn[i] = mod - (mod / i * rn[mod % i]) % mod;

  for (int i = 2; i <= n; ++i) {
    f[i] = f[i - 1] * i % mod;
    inv[i] = inv[i - 1] * rn[i] % mod;
  }

  ll res = 0;
  for (int i = 0; i <= n; i++)
    if ((k - a * i) % b == 0) {
      ll j = (k - a * i) / b;
      if (j <= n) {
        res = (res + C(n, j) * C(n, i) % mod) % mod;
      }
    }
  cout << res << '\n';

  return Accepted;
}

// B...a

Fixed submission:
#include <bits/stdc++.h>

using namespace std;
#define _USE_MATH_DEFINES_
#define ll long long
#define ld long double
#define Accepted 0
#define pb push_back
#define mp make_pair
#define sz(x) (int)(x.size())
#define every(x) x.begin(), x.end()
#define F first
#define S second
#define lb lower_bound
#define ub upper_bound
#define For(i, x, y) for (ll i = x; i <= y; i++)
#define FOr(i, x, y) for (ll i = x; i >= y; i--)
#define SpeedForce ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)
// ROAD to... Red

inline void Input_Output() {
  // freopen(".in", "r", stdin);
  // freopen(".out", "w", stdout);
}

const double eps = 0.000001;
const ld pi = acos(-1);
const int maxn = 1e7 + 9;
const int mod = 998244353;
const ll MOD = 1e18 + 9;
const ll INF = 1e18 + 123;
const int inf = 2e9 + 11;
const int mxn = 1e6 + 9;
const int N = 6e5 + 123;
const int M = 22;
const int pri = 997;
const int Magic = 2101;

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};

ll n, a, b, k;
ll f[N];
ll rn[N];
ll inv[N];
ll left[N];

ll C(int n, int k) {
  if (k > n)
    return 0;
  return (f[n] * inv[k] % mod) * inv[n - k] % mod;
}

ll bpow(ll a, ll b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1)
      res = (res * a) % mod;
    a = (a * a) % mod;
    b /= 2;
  }
  return res;
}

int main() {
  SpeedForce;

  cin >> n >> a >> b >> k;

  f[0] = 1;
  rn[1] = 1;
  inv[0] = 1;
  f[1] = inv[1] = 1;
  for (int i = 2; i <= n; ++i)
    rn[i] = mod - (mod / i * rn[mod % i]) % mod;

  for (int i = 2; i <= n; ++i) {
    f[i] = f[i - 1] * i % mod;
    inv[i] = inv[i - 1] * rn[i] % mod;
  }

  ll res = 0;
  for (int i = 0; i <= n; i++)
    if (k >= a * i && (k - a * i) % b == 0) {
      ll j = (k - a * i) / b;
      if (j <= n) {
        res = (res + C(n, j) * C(n, i) % mod) % mod;
      }
    }
  cout << res << '\n';

  return Accepted;
}

// B...a