Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, a) for (int i = 0; i < (a); i++)
typedef long long ll;

#ifdef _DEBUG
inline void dump() { cerr << endl; }
template <typename Head> void dump(Head &&head) {
  cerr << head;
  dump();
}
template <typename Head, typename... Tail>
void dump(Head &&head, Tail &&...tail) {
  cerr << head << ", ";
  dump(forward<Tail>(tail)...);
}
#define debug(...)                                                             \
  do {                                                                         \
    cerr << __LINE__ << ":\t" << #__VA_ARGS__ << " = ";                        \
    dump(__VA_ARGS__);                                                         \
  } while (false)
#else
#define dump(...)
#define debug(...)
#endif

template <typename T> struct edge {
  int src, to;
  T cost;
  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
  edge &operator=(const int &x) {
    to = x;
    return *this;
  }
  operator int() const { return to; }
};
template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

/////////////////////////////////////////////////////////////////////
const ll inf = 1LL << 60;
const ll mod = 1000000007;

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-=(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;
  }
  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; }
  bool operator==(const mint a) const { return x == a.x; }
  bool operator!=(const mint a) const { return x != a.x; }
  friend ostream &operator<<(ostream &os, const mint &p) { return os << p.x; }
  friend istream &operator>>(istream &is, mint &a) {
    ll t;
    is >> t;
    a = mint(t);
    return (is);
  }
};

template <typename T> struct Comb {
  vector<T> fact, finv, inv;
  Comb(int n) : fact(n, 1), finv(n, 1), inv(n, 1) { init(n); }
  void init(int n) {
    fact.assign(n, 1), finv.assign(n, 1), inv.assign(n, 1);
    for (ll i = 2; i < n; i++) {
      fact[i] = fact[i - 1] * i % mod;
      inv[i] = mod - inv[mod % i] * (mod / i) % mod;
      finv[i] = finv[i - 1] * inv[i] % mod;
    }
  }
  T nck(int n, int k) {
    if (n < k || n < 0 || k < 0)
      return 0;
    return fact[n] * (finv[k] * finv[n - k] % mod) % mod;
  }
  T nhk(int n, int k) { return nck(n + k - 1, k); }
  T npk(int n, int k) {
    if (n < k || n < 0 || k < 0)
      return 0;
    return fact[n] * finv[n - k] % mod;
  }
};

int main() {
  ll k;
  cin >> k;
  string s;
  cin >> s;

  Comb<ll> c(1000010);

  ll n = s.size();

  mint ans = 0;

  rep(i, k + 1) {
    mint tmp = mint(25).pow(i) * c.nhk(n, i);
    tmp *= mint(26).pow(k - i);
    ans += tmp;
  }
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, a) for (int i = 0; i < (a); i++)
typedef long long ll;

#ifdef _DEBUG
inline void dump() { cerr << endl; }
template <typename Head> void dump(Head &&head) {
  cerr << head;
  dump();
}
template <typename Head, typename... Tail>
void dump(Head &&head, Tail &&...tail) {
  cerr << head << ", ";
  dump(forward<Tail>(tail)...);
}
#define debug(...)                                                             \
  do {                                                                         \
    cerr << __LINE__ << ":\t" << #__VA_ARGS__ << " = ";                        \
    dump(__VA_ARGS__);                                                         \
  } while (false)
#else
#define dump(...)
#define debug(...)
#endif

template <typename T> struct edge {
  int src, to;
  T cost;
  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
  edge &operator=(const int &x) {
    to = x;
    return *this;
  }
  operator int() const { return to; }
};
template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

/////////////////////////////////////////////////////////////////////
const ll inf = 1LL << 60;
const ll mod = 1000000007;

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-=(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;
  }
  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; }
  bool operator==(const mint a) const { return x == a.x; }
  bool operator!=(const mint a) const { return x != a.x; }
  friend ostream &operator<<(ostream &os, const mint &p) { return os << p.x; }
  friend istream &operator>>(istream &is, mint &a) {
    ll t;
    is >> t;
    a = mint(t);
    return (is);
  }
};

template <typename T> struct Comb {
  vector<T> fact, finv, inv;
  Comb(int n) : fact(n, 1), finv(n, 1), inv(n, 1) { init(n); }
  void init(int n) {
    fact.assign(n, 1), finv.assign(n, 1), inv.assign(n, 1);
    for (ll i = 2; i < n; i++) {
      fact[i] = fact[i - 1] * i % mod;
      inv[i] = mod - inv[mod % i] * (mod / i) % mod;
      finv[i] = finv[i - 1] * inv[i] % mod;
    }
  }
  T nck(int n, int k) {
    if (n < k || n < 0 || k < 0)
      return 0;
    return fact[n] * (finv[k] * finv[n - k] % mod) % mod;
  }
  T nhk(int n, int k) { return nck(n + k - 1, k); }
  T npk(int n, int k) {
    if (n < k || n < 0 || k < 0)
      return 0;
    return fact[n] * finv[n - k] % mod;
  }
};

int main() {
  ll k;
  cin >> k;
  string s;
  cin >> s;

  Comb<ll> c(2000010);

  ll n = s.size();

  mint ans = 0;

  rep(i, k + 1) {
    mint tmp = mint(25).pow(i) * c.nhk(n, i);
    tmp *= mint(26).pow(k - i);
    ans += tmp;
  }
  cout << ans << endl;

  return 0;
}