Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h> // clang-format off
using Int = long long;
#define REP_(i, a_, b_, a, b, ...) for (Int i = (a), lim##i = (b); i < lim##i; i++)
#define REP(i, ...) REP_(i, __VA_ARGS__, __VA_ARGS__, 0, __VA_ARGS__)
struct SetupIO { SetupIO() { std::cin.tie(nullptr), std::ios::sync_with_stdio(false), std::cout << std::fixed << std::setprecision(13); } } setup_io;
#ifndef dump
#define dump(...)
#endif // clang-format on

/**
 *    author:  knshnb
 *    created: Sun Jun 21 21:22:27 JST 2020
 **/

template <class T> T pow(T x, long long n, const T UNION = 1) {
  T ret = UNION;
  while (n) {
    if (n & 1)
      ret *= x;
    x *= x;
    n >>= 1;
  }
  return ret;
}

/// @docs src/Math/ModInt.md
template <int Mod> struct ModInt {
  int x;
  static int runtime_mod;
  static std::unordered_map<int, int> to_inv;
  // テンプレート引数が負のときは実行時ModInt
  static int mod() { return Mod < 0 ? runtime_mod : Mod; }
  static void set_runtime_mod(int mod) {
    static_assert(Mod < 0,
                  "template parameter Mod must be negative for runtime ModInt");
    runtime_mod = mod;
    to_inv.clear();
  }
  ModInt() : x(0) {}
  ModInt(long long x_) {
    if ((x = x_ % mod() + mod()) >= mod())
      x -= mod();
  }

  ModInt &operator+=(ModInt rhs) {
    if ((x += rhs.x) >= mod())
      x -= mod();
    return *this;
  }
  ModInt &operator*=(ModInt rhs) {
    x = (unsigned long long)x * rhs.x % mod();
    return *this;
  }
  ModInt &operator-=(ModInt rhs) {
    if ((x -= rhs.x) < 0)
      x += mod();
    return *this;
  }
  ModInt &operator/=(ModInt rhs) {
    x = (unsigned long long)x * rhs.inv().x % mod();
    return *this;
  }
  ModInt operator-() const { return -x < 0 ? mod() - x : -x; }
  ModInt operator+(ModInt rhs) const { return ModInt(*this) += rhs; }
  ModInt operator*(ModInt rhs) const { return ModInt(*this) *= rhs; }
  ModInt operator-(ModInt rhs) const { return ModInt(*this) -= rhs; }
  ModInt operator/(ModInt rhs) const { return ModInt(*this) /= rhs; }
  bool operator==(ModInt rhs) const { return x == rhs.x; }
  bool operator!=(ModInt rhs) const { return x != rhs.x; }
  ModInt inv() const {
    return to_inv.count(this->x) ? to_inv[this->x]
                                 : (to_inv[this->x] = pow(*this, mod() - 2).x);
  }

  friend std::ostream &operator<<(std::ostream &s, ModInt<Mod> a) {
    s << a.x;
    return s;
  }
  friend std::istream &operator>>(std::istream &s, ModInt<Mod> &a) {
    long long tmp;
    s >> tmp;
    a = ModInt<Mod>(tmp);
    return s;
  }
  friend std::string to_string(ModInt<Mod> a) { return std::to_string(a.x); }
};
template <int Mod> std::unordered_map<int, int> ModInt<Mod>::to_inv;
template <int Mod> int ModInt<Mod>::runtime_mod;

#ifndef CALL_FROM_TEST
using mint = ModInt<1000000007>;
#endif

template <class T> struct Combination {
  std::vector<T> fact, fact_inv;
  Combination(int n = 1000003) : fact(n + 1, 1), fact_inv(n + 1) {
    for (int i = 0; i < n; i++)
      fact[i + 1] = fact[i] * (i + 1);
    fact_inv[n] = (T)1 / fact[n];
    for (int i = n - 1; i >= 0; i--)
      fact_inv[i] = fact_inv[i + 1] * (i + 1);
    // for (int i = 0; i < n + 1; i++) assert(fact[i] * fact_inv[i] == 1);
  }
  T operator()(int n, int r) { return fact[n] * fact_inv[r] * fact_inv[n - r]; }
};

signed main() {
  Combination<mint> comb;
  Int K;
  std::string s;
  std::cin >> K >> s;
  Int n = s.size();
  mint ans = 0;
  REP(i, n, n + K + 1) { ans += comb(n + K, i) * pow(mint(25), n + K - i); }
  std::cout << ans << std::endl;
}


Fixed submission:
#include <bits/stdc++.h> // clang-format off
using Int = long long;
#define REP_(i, a_, b_, a, b, ...) for (Int i = (a), lim##i = (b); i < lim##i; i++)
#define REP(i, ...) REP_(i, __VA_ARGS__, __VA_ARGS__, 0, __VA_ARGS__)
struct SetupIO { SetupIO() { std::cin.tie(nullptr), std::ios::sync_with_stdio(false), std::cout << std::fixed << std::setprecision(13); } } setup_io;
#ifndef dump
#define dump(...)
#endif // clang-format on

/**
 *    author:  knshnb
 *    created: Sun Jun 21 21:22:27 JST 2020
 **/

template <class T> T pow(T x, long long n, const T UNION = 1) {
  T ret = UNION;
  while (n) {
    if (n & 1)
      ret *= x;
    x *= x;
    n >>= 1;
  }
  return ret;
}

/// @docs src/Math/ModInt.md
template <int Mod> struct ModInt {
  int x;
  static int runtime_mod;
  static std::unordered_map<int, int> to_inv;
  // テンプレート引数が負のときは実行時ModInt
  static int mod() { return Mod < 0 ? runtime_mod : Mod; }
  static void set_runtime_mod(int mod) {
    static_assert(Mod < 0,
                  "template parameter Mod must be negative for runtime ModInt");
    runtime_mod = mod;
    to_inv.clear();
  }
  ModInt() : x(0) {}
  ModInt(long long x_) {
    if ((x = x_ % mod() + mod()) >= mod())
      x -= mod();
  }

  ModInt &operator+=(ModInt rhs) {
    if ((x += rhs.x) >= mod())
      x -= mod();
    return *this;
  }
  ModInt &operator*=(ModInt rhs) {
    x = (unsigned long long)x * rhs.x % mod();
    return *this;
  }
  ModInt &operator-=(ModInt rhs) {
    if ((x -= rhs.x) < 0)
      x += mod();
    return *this;
  }
  ModInt &operator/=(ModInt rhs) {
    x = (unsigned long long)x * rhs.inv().x % mod();
    return *this;
  }
  ModInt operator-() const { return -x < 0 ? mod() - x : -x; }
  ModInt operator+(ModInt rhs) const { return ModInt(*this) += rhs; }
  ModInt operator*(ModInt rhs) const { return ModInt(*this) *= rhs; }
  ModInt operator-(ModInt rhs) const { return ModInt(*this) -= rhs; }
  ModInt operator/(ModInt rhs) const { return ModInt(*this) /= rhs; }
  bool operator==(ModInt rhs) const { return x == rhs.x; }
  bool operator!=(ModInt rhs) const { return x != rhs.x; }
  ModInt inv() const {
    return to_inv.count(this->x) ? to_inv[this->x]
                                 : (to_inv[this->x] = pow(*this, mod() - 2).x);
  }

  friend std::ostream &operator<<(std::ostream &s, ModInt<Mod> a) {
    s << a.x;
    return s;
  }
  friend std::istream &operator>>(std::istream &s, ModInt<Mod> &a) {
    long long tmp;
    s >> tmp;
    a = ModInt<Mod>(tmp);
    return s;
  }
  friend std::string to_string(ModInt<Mod> a) { return std::to_string(a.x); }
};
template <int Mod> std::unordered_map<int, int> ModInt<Mod>::to_inv;
template <int Mod> int ModInt<Mod>::runtime_mod;

#ifndef CALL_FROM_TEST
using mint = ModInt<1000000007>;
#endif

template <class T> struct Combination {
  std::vector<T> fact, fact_inv;
  Combination(int n = 2000003) : fact(n + 1, 1), fact_inv(n + 1) {
    for (int i = 0; i < n; i++)
      fact[i + 1] = fact[i] * (i + 1);
    fact_inv[n] = (T)1 / fact[n];
    for (int i = n - 1; i >= 0; i--)
      fact_inv[i] = fact_inv[i + 1] * (i + 1);
    // for (int i = 0; i < n + 1; i++) assert(fact[i] * fact_inv[i] == 1);
  }
  T operator()(int n, int r) { return fact[n] * fact_inv[r] * fact_inv[n - r]; }
};

signed main() {
  Combination<mint> comb;
  Int K;
  std::string s;
  std::cin >> K >> s;
  Int n = s.size();
  mint ans = 0;
  REP(i, n, n + K + 1) { ans += comb(n + K, i) * pow(mint(25), n + K - i); }
  std::cout << ans << std::endl;
}