Problem p02685 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i < (n); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define maxs(a, b) a = max(a, b)
#define mins(a, b) a = min(a, b)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const ll linf = (1LL << 61);
const int inf = 1001001001;
// const int mod = 1000000007;
const int mod = 998244353;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  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 { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= 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 { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
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];
  }
} c(200005);

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ll n, m, k;
  cin >> n >> m >> k;
  mint ans = 0;
  mint col = m;
  for (int i = n - 1; i >= 0; --i) {
    mint now = col;
    now *= c(n - 1, i);
    if (i <= k)
      ans += now;
    col *= m - 1;
  }
  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 rrep(i, n) for (int i = 1; i < (n); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define maxs(a, b) a = max(a, b)
#define mins(a, b) a = min(a, b)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const ll linf = (1LL << 61);
const int inf = 1001001001;
// const int mod = 1000000007;
const int mod = 998244353;

ll modPow(long long a, long long n, long long p) { // 0でバグる
  if (n == 1)
    return a % p;
  if (n % 2 == 1)
    return (a * modPow(a, n - 1, p)) % p;
  long long t = modPow(a, n / 2, p);
  return (t * t) % p;
}

ll comb(ll n, ll p) {
  if (p * 2 > n)
    p = n - p;
  if (p > n - p)
    p = n - p;
  ll a = 1, b = 1;
  rrep(i, p + 1) {
    a = (a * (n - i + 1)) % mod;
    b = (b * i) % mod;
  }
  return a * (modPow(b, mod - 2, mod)) % mod;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ll n, m, k;
  cin >> n >> m >> k;
  ll ans = 0;
  ans += m * modPow(m - 1, n - 1, mod);
  for (int i = 1; i <= k; ++i) { // 同じ組をi組作る
    ll tmp = 0;
    (tmp += (m * comb(n - 1, i) % mod) * modPow(m - 1, n - 1 - i, mod)) %= mod;
    (ans += tmp) %= mod;
  }
  cout << ans % mod << endl;
  return 0;
}