Problem p02685 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#include <cmath>
using ll = long long;
using namespace std;
stack<int> st;
queue<int> qu;
queue<pair<int, int>> qu2;
priority_queue<int> pq;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, n) for (int i = 1; i <= (int)(n); i++)
#define mins(x, y) x = min(x, y)
#define maxs(x, y) x = max(x, y)
#define ALL(a) a.begin(), a.end()
typedef set<int> set_t;
typedef set<string> set_g;
typedef complex<double> xy_t;
static const int NIL = -1;
static const int INF = 1000000007;
#define mp make_pair
#define sz(x) int(x.sise())

#define reps(i, s, n) for (int i = s; i < n; i++)
#define Rreps(i, n, e) for (int i = n - 1; i >= e; --i)
#define Rrep(i, n) Rreps(i, n, 0)
deque<int> deq;
#define fi first
#define se second
// const ll MOD = 998244353;
const ll MOD = (1e+9) + 7;
typedef pair<int, int> P;

// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
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; }

// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
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() {
  int n, m, k;
  cin >> n >> m >> k;
  mint color = m;
  rep(i, n - k) { color *= (m - 1); }
  mint ans = color * c(n + k - 1, k);
  cout << ans.x << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#include <cmath>
using ll = long long;
using namespace std;
stack<int> st;
queue<int> qu;
queue<pair<int, int>> qu2;
priority_queue<int> pq;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, n) for (int i = 1; i <= (int)(n); i++)
#define mins(x, y) x = min(x, y)
#define maxs(x, y) x = max(x, y)
#define ALL(a) a.begin(), a.end()
typedef set<int> set_t;
typedef set<string> set_g;
typedef complex<double> xy_t;
static const int NIL = -1;
static const int INF = 1000000007;
#define mp make_pair
#define sz(x) int(x.sise())

#define reps(i, s, n) for (int i = s; i < n; i++)
#define Rreps(i, n, e) for (int i = n - 1; i >= e; --i)
#define Rrep(i, n) Rreps(i, n, 0)
deque<int> deq;
#define fi first
#define se second
// const ll MOD = 998244353;
const ll MOD = (1e+9) + 7;
typedef pair<int, int> P;

// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
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; }

// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
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() {
  int n, m, k;
  cin >> n >> m >> k;
  mint ans = 0;
  mint col = m;
  for (int x = n - 1; x >= 0; --x) {
    mint now = col;
    now *= c(n - 1, x);
    if (x <= k)
      ans += now;
    col *= m - 1;
  }
  cout << ans << endl;
  return 0;
}