Problem p02632 (C++)

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

// template {{{
#define range(i, l, r) for (int i = (int)(l); i < (int)(r); (i) += 1)
#define rrange(i, l, r) for (int i = (int)(r)-1; i >= (int)(l); (i) -= 1)

#define whole(f, x, ...)                                                       \
  ([&](decltype((x)) container) {                                              \
    return (f)(begin(container), end(container), ##__VA_ARGS__);               \
  })(x)
#define rwhole(f, x, ...)                                                      \
  ([&](decltype((x)) container) {                                              \
    return (f)(rbegin(container), rend(container), ##__VA_ARGS__);             \
  })(x)

#define debug(x) cerr << "(" << __LINE__ << ")" << #x << ": " << (x) << endl

using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;

// constexpr i32 mod   = 998244353;
constexpr i32 mod = 1e9 + 7;
constexpr i32 inf = 1001001001;
constexpr i64 infll = 1001001001001001001ll;

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

struct IoSetup {
  IoSetup(int x = 15) {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(x);
    cerr << fixed << setprecision(x);
  }
} iosetup;

template <typename T = i64> T input() {
  T x;
  cin >> x;
  return x;
}

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  range(i, 0, v.size()) { os << v[i] << (i + 1 != v.size() ? " " : ""); }
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> vector<T> make_vector(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_vector(size_t a, Ts... ts) {
  return vector<decltype(make_vector(ts...))>(a, make_vector(ts...));
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}
// }}}

// mod_int {{{
template <int mod> class mod_int {
  using i32 = int_fast32_t;
  using i64 = int_fast64_t;

  i32 num_;

public:
  mod_int() : num_(0) {}
  mod_int(i64 num_) : num_(num_ >= 0 ? num_ % mod : (mod + num_ % mod) % mod) {}

  inline i32 num() const { return num_; }
  operator i64() { return num_; }

  inline mod_int inverse() const {
    i32 a = num_, b = mod, u = 1, v = 0, t;
    while (b) {
      t = a / b;
      std::swap(a -= t * b, b);
      std::swap(u -= t * v, v);
    }
    return mod_int(u);
  }

  mod_int operator~() const { return inverse(); }
  mod_int operator+() const { return *this; }
  mod_int operator-() const { return mod_int(-num_); }

  mod_int &operator+=(const mod_int &rhs) {
    if ((num_ += rhs.num_) >= mod)
      num_ -= mod;
    return *this;
  }
  mod_int &operator-=(const mod_int &rhs) {
    if ((num_ -= rhs.num_) < 0)
      num_ += mod;
    return *this;
  }
  mod_int &operator*=(const mod_int &rhs) {
    num_ = i64(num_) * rhs.num_ % mod;
    return *this;
  }
  mod_int &operator/=(const mod_int &rhs) { return *this *= rhs.inverse(); }

  mod_int operator+(const mod_int &rhs) const { return mod_int(*this) += rhs; }
  mod_int operator-(const mod_int &rhs) const { return mod_int(*this) -= rhs; }
  mod_int operator*(const mod_int &rhs) const { return mod_int(*this) *= rhs; }
  mod_int operator/(const mod_int &rhs) const { return mod_int(*this) /= rhs; }

  bool operator==(const mod_int &rhs) const { return num_ == rhs.num_; }
  bool operator!=(const mod_int &rhs) const { return num_ != rhs.num_; }

  friend std::ostream &operator<<(std::ostream &os, const mod_int &rhs) {
    return os << rhs.num_;
  }

  friend std::istream &operator>>(std::istream &is, mod_int &rhs) {
    i64 num;
    is >> num;
    rhs = mod_int<mod>(num);
    return is;
  }

  friend mod_int pow(mod_int lhs, i64 rhs) {
    mod_int res(1);
    for (; rhs; rhs >>= 1) {
      if (rhs & 1)
        res *= lhs;
      lhs *= lhs;
    }
    return res;
  }
};
// }}}

using modint = mod_int<mod>;

// mod combination {{{
template <typename T> class mod_combinatorics {

  std::size_t size_;
  std::vector<T> fact_, finv_, invs_;

public:
  mod_combinatorics(std::size_t size)
      : size_(size + 1), fact_(size_), finv_(size_), invs_(size_) {
    fact_[0] = 1;
    for (int i = 1; i < size_; ++i)
      fact_[i] = fact_[i - 1] * T(i);

    finv_.back() = fact_.back().inverse();
    for (int i = int_fast32_t(size_) - 2; i >= 0; --i)
      finv_[i] = finv_[i + 1] * T(i + 1);
    for (int i = 0; i < size; ++i)
      invs_[i] = finv_[i] * fact_[i - 1];
  }

  inline void boundary_check(int n) const {
    if (size_ < n) {
      std::cerr << "boudary error" << std::endl;
    }
  }

  // k!
  T fact(int k) const {
    boundary_check(k);
    return fact_[k];
  }

  // P(n, k)
  T permutation(int n, int k) const {
    boundary_check(n);
    if (k < 0 || n < k)
      return T(0);
    return fact_[n] * finv_[n - k];
  }

  // C(n, k)
  T combination(int n, int k) const {
    boundary_check(n);
    if (k < 0 || n < k)
      return T(0);
    return fact_[n] * finv_[k] * finv_[n - k];
  }

  // H(n, k)
  T multichoose(int n, int k) const {
    boundary_check(n + k - 1);
    if (n < 0 || k < 0)
      return T(0);
    return fact_[n + k - 1] * finv_[k] * finv_[n - 1];
  }
};
// }}}

void solver() {
  int k = input();
  string s;
  cin >> s;

  vector<int> cnt(26);
  for (auto c : s)
    cnt[c - 'a']++;

  mod_combinatorics<modint> mc(300000);

  modint ans = 0;
  range(i, s.size(), s.size() + k + 1) {
    ans += mc.combination(i - 1, s.size() - 1) * pow(modint(25), i - s.size()) *
           pow(modint(26), s.size() + k - i);
  }

  cout << ans << endl;
}

signed main(int argc, char *argv[]) { solver(); }


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

// template {{{
#define range(i, l, r) for (int i = (int)(l); i < (int)(r); (i) += 1)
#define rrange(i, l, r) for (int i = (int)(r)-1; i >= (int)(l); (i) -= 1)

#define whole(f, x, ...)                                                       \
  ([&](decltype((x)) container) {                                              \
    return (f)(begin(container), end(container), ##__VA_ARGS__);               \
  })(x)
#define rwhole(f, x, ...)                                                      \
  ([&](decltype((x)) container) {                                              \
    return (f)(rbegin(container), rend(container), ##__VA_ARGS__);             \
  })(x)

#define debug(x) cerr << "(" << __LINE__ << ")" << #x << ": " << (x) << endl

using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;

// constexpr i32 mod   = 998244353;
constexpr i32 mod = 1e9 + 7;
constexpr i32 inf = 1001001001;
constexpr i64 infll = 1001001001001001001ll;

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

struct IoSetup {
  IoSetup(int x = 15) {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(x);
    cerr << fixed << setprecision(x);
  }
} iosetup;

template <typename T = i64> T input() {
  T x;
  cin >> x;
  return x;
}

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  range(i, 0, v.size()) { os << v[i] << (i + 1 != v.size() ? " " : ""); }
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> vector<T> make_vector(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_vector(size_t a, Ts... ts) {
  return vector<decltype(make_vector(ts...))>(a, make_vector(ts...));
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}
// }}}

// mod_int {{{
template <int mod> class mod_int {
  using i32 = int_fast32_t;
  using i64 = int_fast64_t;

  i32 num_;

public:
  mod_int() : num_(0) {}
  mod_int(i64 num_) : num_(num_ >= 0 ? num_ % mod : (mod + num_ % mod) % mod) {}

  inline i32 num() const { return num_; }
  operator i64() { return num_; }

  inline mod_int inverse() const {
    i32 a = num_, b = mod, u = 1, v = 0, t;
    while (b) {
      t = a / b;
      std::swap(a -= t * b, b);
      std::swap(u -= t * v, v);
    }
    return mod_int(u);
  }

  mod_int operator~() const { return inverse(); }
  mod_int operator+() const { return *this; }
  mod_int operator-() const { return mod_int(-num_); }

  mod_int &operator+=(const mod_int &rhs) {
    if ((num_ += rhs.num_) >= mod)
      num_ -= mod;
    return *this;
  }
  mod_int &operator-=(const mod_int &rhs) {
    if ((num_ -= rhs.num_) < 0)
      num_ += mod;
    return *this;
  }
  mod_int &operator*=(const mod_int &rhs) {
    num_ = i64(num_) * rhs.num_ % mod;
    return *this;
  }
  mod_int &operator/=(const mod_int &rhs) { return *this *= rhs.inverse(); }

  mod_int operator+(const mod_int &rhs) const { return mod_int(*this) += rhs; }
  mod_int operator-(const mod_int &rhs) const { return mod_int(*this) -= rhs; }
  mod_int operator*(const mod_int &rhs) const { return mod_int(*this) *= rhs; }
  mod_int operator/(const mod_int &rhs) const { return mod_int(*this) /= rhs; }

  bool operator==(const mod_int &rhs) const { return num_ == rhs.num_; }
  bool operator!=(const mod_int &rhs) const { return num_ != rhs.num_; }

  friend std::ostream &operator<<(std::ostream &os, const mod_int &rhs) {
    return os << rhs.num_;
  }

  friend std::istream &operator>>(std::istream &is, mod_int &rhs) {
    i64 num;
    is >> num;
    rhs = mod_int<mod>(num);
    return is;
  }

  friend mod_int pow(mod_int lhs, i64 rhs) {
    mod_int res(1);
    for (; rhs; rhs >>= 1) {
      if (rhs & 1)
        res *= lhs;
      lhs *= lhs;
    }
    return res;
  }
};
// }}}

using modint = mod_int<mod>;

// mod combination {{{
template <typename T> class mod_combinatorics {

  std::size_t size_;
  std::vector<T> fact_, finv_, invs_;

public:
  mod_combinatorics(std::size_t size)
      : size_(size + 1), fact_(size_), finv_(size_), invs_(size_) {
    fact_[0] = 1;
    for (int i = 1; i < size_; ++i)
      fact_[i] = fact_[i - 1] * T(i);

    finv_.back() = fact_.back().inverse();
    for (int i = int_fast32_t(size_) - 2; i >= 0; --i)
      finv_[i] = finv_[i + 1] * T(i + 1);
    for (int i = 0; i < size; ++i)
      invs_[i] = finv_[i] * fact_[i - 1];
  }

  inline void boundary_check(int n) const {
    if (size_ < n) {
      std::cerr << "boudary error" << std::endl;
    }
  }

  // k!
  T fact(int k) const {
    boundary_check(k);
    return fact_[k];
  }

  // P(n, k)
  T permutation(int n, int k) const {
    boundary_check(n);
    if (k < 0 || n < k)
      return T(0);
    return fact_[n] * finv_[n - k];
  }

  // C(n, k)
  T combination(int n, int k) const {
    boundary_check(n);
    if (k < 0 || n < k)
      return T(0);
    return fact_[n] * finv_[k] * finv_[n - k];
  }

  // H(n, k)
  T multichoose(int n, int k) const {
    boundary_check(n + k - 1);
    if (n < 0 || k < 0)
      return T(0);
    return fact_[n + k - 1] * finv_[k] * finv_[n - 1];
  }
};
// }}}

void solver() {
  int k = input();
  string s;
  cin >> s;

  mod_combinatorics<modint> mc(3000000);

  modint ans = 0;
  range(i, s.size(), s.size() + k + 1) {
    ans += mc.combination(i - 1, s.size() - 1) * pow(modint(25), i - s.size()) *
           pow(modint(26), s.size() + k - i);
  }

  cout << ans << endl;
}

signed main(int argc, char *argv[]) { solver(); }