Problem p02632 (C++)

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

#define lli long long int
#define REP(i, s, n) for (int i = s; i < n; i++)
#define NUM 2520
#define INF (1LL << 50)
#define DEBUG 1
#define mp(a, b) make_pair(a, b)
#define SORT(V) sort(V.begin(), V.end())
#define PI (3.141592653589794)
#define TO_STRING(VariableName) #VariableName
#define LOG(x)                                                                 \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << endl;
#define LOG2(x, y)                                                             \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << TO_STRING(y) << "=" << y << endl;
#define LOG3(x, y, z)                                                          \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << TO_STRING(y) << "=" << y << " " \
         << TO_STRING(z) << "=" << z << endl;
#define LOG4(w, x, y, z)                                                       \
  if (DEBUG)                                                                   \
    cout << TO_STRING(w) << "=" << w << " " << TO_STRING(x) << "=" << x << " " \
         << TO_STRING(y) << "=" << y << " " << TO_STRING(z) << "=" << z        \
         << endl;

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

const long long MOD = 1000000007;

// modint: mod 計算を int を扱うように扱える構造体
template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

// a^n mod を計算する
lli modpow(lli a, lli n, lli mod) {
  lli res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

// a^-1
lli modinv(lli a, lli mod) {
  lli b = mod, u = 1, v = 0;
  while (b) {
    lli t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= mod;
  if (u < 0)
    u += mod;
  return u;
}

// const int MOD = 1000000007;
using mint = Fp<MOD>;

const lli MAX = 1000100;

lli fac[MAX], finv[MAX], inv[MAX];
/*use method: COMinit(),COM(x,y)*/
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (lli i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(lli n, lli k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

void func(long long K, std::string S) {

  mint ans = 0;
  REP(i, 0, K + 1) {
    mint now = COM(i + S.size() - 1, S.size() - 1);
    now *= modpow(26, K - i, MOD);
    now *= modpow(25, i, MOD);
    ans += now;
  }

  cout << ans << endl;
}

int main() {
  // cout << fixed << setprecision(5);

  COMinit();
  long long K;
  scanf("%lld", &K);
  std::string S;
  std::cin >> S;
  func(K, S);
  return 0;
}


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

#define lli long long int
#define REP(i, s, n) for (int i = s; i < n; i++)
#define NUM 2520
#define INF (1LL << 50)
#define DEBUG 1
#define mp(a, b) make_pair(a, b)
#define SORT(V) sort(V.begin(), V.end())
#define PI (3.141592653589794)
#define TO_STRING(VariableName) #VariableName
#define LOG(x)                                                                 \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << endl;
#define LOG2(x, y)                                                             \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << TO_STRING(y) << "=" << y << endl;
#define LOG3(x, y, z)                                                          \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << TO_STRING(y) << "=" << y << " " \
         << TO_STRING(z) << "=" << z << endl;
#define LOG4(w, x, y, z)                                                       \
  if (DEBUG)                                                                   \
    cout << TO_STRING(w) << "=" << w << " " << TO_STRING(x) << "=" << x << " " \
         << TO_STRING(y) << "=" << y << " " << TO_STRING(z) << "=" << z        \
         << endl;

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

const long long MOD = 1000000007;

// modint: mod 計算を int を扱うように扱える構造体
template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

// a^n mod を計算する
lli modpow(lli a, lli n, lli mod) {
  lli res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

// a^-1
lli modinv(lli a, lli mod) {
  lli b = mod, u = 1, v = 0;
  while (b) {
    lli t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= mod;
  if (u < 0)
    u += mod;
  return u;
}

// const int MOD = 1000000007;
using mint = Fp<MOD>;

const lli MAX = 2000100;

lli fac[MAX], finv[MAX], inv[MAX];
/*use method: COMinit(),COM(x,y)*/
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (lli i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(lli n, lli k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

void func(long long K, std::string S) {

  mint ans = 0;
  REP(i, 0, K + 1) {
    mint now = COM(i + S.size() - 1, S.size() - 1);
    now *= modpow(26, K - i, MOD);
    now *= modpow(25, i, MOD);
    ans += now;
  }

  cout << ans << endl;
}

int main() {
  // cout << fixed << setprecision(5);

  COMinit();
  long long K;
  scanf("%lld", &K);
  std::string S;
  std::cin >> S;
  func(K, S);
  return 0;
}