Problem p02632 (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 = (n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (n); ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.first >> p.second;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << '{' << p.first << "," << p.second << '}';
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? ']' : ',');
  return os;
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;

const int mod = 1000000007;
// const int mod = 998244353;
struct mint {
  ll x;
  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++() { return *this += mint(1); }
  mint &operator-=(const mint &a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator--() { return *this -= mint(1); }
  mint &operator*=(const mint &a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    mint res = mint(1), a(*this);
    while (t > 0) {
      if (t & 1)
        res *= a;
      t >>= 1;
      a *= a;
    }
    return res;
  }
  // 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 {
    mint res(*this);
    return res /= a;
  }
};
ostream &operator<<(ostream &os, const mint &m) { return os << m.x; }

class combination {
  vector<mint> fact, ifact;

public:
  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];
  }
} comb(2000010);

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int k;
  string s;
  cin >> k >> s;
  int n = s.size();
  mint ans = 0;
  rep(i, k + 1) {
    ans += mint(26).pow(i) * mint(25).pow(k - i) * comb(n + k - i - 1, n - 1);
  }
  cout << ans << endl;
}

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 = (n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (n); ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.first >> p.second;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << '{' << p.first << "," << p.second << '}';
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? ']' : ',');
  return os;
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;

const int mod = 1000000007;
// const int mod = 998244353;
struct mint {
  ll x;
  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++() { return *this += mint(1); }
  mint &operator-=(const mint &a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator--() { return *this -= mint(1); }
  mint &operator*=(const mint &a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    mint res = mint(1), a(*this);
    while (t > 0) {
      if (t & 1)
        res *= a;
      t >>= 1;
      a *= a;
    }
    return res;
  }
  // 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 {
    mint res(*this);
    return res /= a;
  }
};
ostream &operator<<(ostream &os, const mint &m) { return os << m.x; }

template <typename T> struct matrix {
  int h, w;
  vector<vector<T>> v;
  matrix(int h, int w, T t)
      : h(h), w(w), v(vector<vector<T>>(h, vector<T>(w, t))) {}
  matrix(vector<vector<T>> v) : v(v), h(v.size()), w(v[0].size()) {}
  matrix &operator+=(const matrix &a) {
    rep(i, h) rep(j, w) v[i][j] += a.v[i][j];
    return *this;
  }
  matrix &operator*=(const int &k) {
    rep(i, h) rep(j, w) v[i][j] *= k;
    return *this;
  }
  matrix &operator-=(const matrix &a) {
    *this += a * (-1);
    return *this;
  }
  matrix operator+(const matrix &a) const { return res(*this) += a; }
  matrix operator*(const int &k) const { return res(*this) *= k; }
  matrix operator-(const matrix &a) const { return res(*this) -= a; }
  matrix operator*(const matrix &a) const {
    matrix res(h, a.w, 0);
    rep(i, h) rep(j, a.h) rep(k, a.w) res.v[i][k] += v[i][j] * a.v[j][k];
    return res;
  }
  matrix &operator*=(const matrix &a) { return *this = *this * a; }
  matrix pow(ll t) const {
    matrix res(h, w, 0), a(*this);
    rep(i, h) res.v[i][i] = 1;
    while (t > 0) {
      if (t & 1)
        res *= a;
      t >>= 1;
      a *= a;
    }
    return res;
  }
};

using vvm = vector<vector<mint>>;
using vm = vector<mint>;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int k;
  string s;
  cin >> k >> s;
  int n = s.size();
  vvm init(n + 1, vm(1, 0));
  init[0][0] = 1;
  vvm mv(n + 1, vm(n + 1, 0));
  rep(i, 26) {
    rep(j, n + 1) {
      if (j < n and s[j] - 'a' == i) {
        mv[j + 1][j] += 1;
      } else {
        mv[j][j] += 1;
      }
    }
  }
  matrix<mint> init_m(init), mv_m(mv);
  auto ans = mv_m.pow(k + n) * init_m;
  cout << ans.v[n][0] << endl;
}