Problem p02632 (C++)

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

using namespace std;

using ll = long long;
using P = pair<ll, ll>;
using Graph = vector<vector<ll>>;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rep2(i, m, n) for (ll i = m; i < (ll)(n); i++)
#define rrep(i, n, m) for (ll i = n; i >= (ll)(m); i--)
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int ddx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int ddy[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll MOD = 1000000007;
const ll INF = 1000000000000000000L;
#ifdef __DEBUG

/**
 * For DEBUG
 * https://github.com/ta7uw/cpp-pyprint
 */
#include "cpp-pyprint/pyprint.h"

#endif

class mint {
public:
  long long x;

  constexpr mint(long long x = 0) : x((x % MOD + MOD) % MOD) {}

  constexpr mint operator-() const { return mint(-x); }

  constexpr mint &operator+=(const mint &rhs) {
    if ((x += rhs.x) >= MOD)
      x -= MOD;
    return *this;
  }

  constexpr mint &operator-=(const mint &rhs) {
    if ((x += MOD - rhs.x) >= MOD)
      x -= MOD;
    return *this;
  }

  constexpr mint &operator*=(const mint &rhs) {
    (x *= rhs.x) %= MOD;
    return *this;
  }

  constexpr mint operator+(const mint &rhs) const {
    mint res(*this);
    return res += rhs;
  }

  constexpr mint operator-(const mint &rhs) const {
    mint res(*this);
    return res -= rhs;
  }

  constexpr mint operator*(const mint &rhs) const {
    mint res(*this);
    return res *= rhs;
  }

  [[nodiscard]] constexpr mint pow(long long t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  [[nodiscard]] constexpr mint inv() const { return pow(MOD - 2); }

  mint &operator/=(mint rhs) { return *this *= rhs.inv(); }

  mint operator/(const mint rhs) const { return mint(*this) /= rhs; }

  constexpr bool operator==(const mint &rhs) const noexcept {
    return this->x == rhs.x;
  }

  constexpr bool operator!=(const mint &rhs) const noexcept {
    return this->x != rhs.x;
  }

  bool operator<(const mint &rhs) const noexcept { return this->x < rhs.x; }

  bool operator>(const mint &rhs) const noexcept { return this->x > rhs.x; }

  bool operator<=(const mint &rhs) const noexcept { return this->x <= rhs.x; }

  bool operator>=(const mint &rhs) const noexcept { return this->x >= rhs.x; }

  friend istream &operator>>(istream &is, mint &a) {
    long long t;
    is >> t;
    a = mint(t);
    return is;
  }

  friend ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
};

const int MAX = 510000;
long long fac[MAX], finv[MAX], inv[MAX];
bool initialized = false;

/**
 * @fn
 * This function must be called at first.
 */
void combination_init() {
  initialized = true;
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int 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;
  }
}

/**
 * @fn
 * Calculates the combination  ( nCK % p: N choose K mod p).
 *
 * @brief constraint: 1 ≤ k ≤ n ≤ 10^7
 * @return nCK % MOD
 */
long long combination(int n, int k) {
  assert(initialized);
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

/**
 * @fn
 * Calculates the factorial of the number.
 *
 * @return N! % MOD
 */
long long factorial(int n) {
  assert(initialized);
  if (n < 0)
    return 0;
  return fac[n] % MOD;
}

/**
 * @fn
 * Calculates the combination ( nCK % p: N choose K mod p).
 *
 * @brief constraint: 1 ≤ k ≤ 10^7 and 1 ≤ n ≤ 10^9
 * @return nCK % MOD
 */
long long combination2(int n, int k) {
  assert(initialized);
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  ll res = 1;
  k = min(k, n - k);
  for (long long x = 1; x <= k; x++) {
    res *= (n - x + 1);
    res %= MOD;
    res *= inv[x];
    res %= MOD;
  }
  return res;
}

void solve() {
  ll K;
  string S;
  combination_init();
  cin >> K >> S;
  mint ans = 0;
  ll N = S.size();
  rep(i, K + 1) {
    ans += mint(25).pow(K - i) * combination(N + K - i - 1, N - 1) *
           mint(26).pow(i);
  }
  cout << ans << '\n';
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  solve();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using P = pair<ll, ll>;
using Graph = vector<vector<ll>>;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rep2(i, m, n) for (ll i = m; i < (ll)(n); i++)
#define rrep(i, n, m) for (ll i = n; i >= (ll)(m); i--)
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int ddx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int ddy[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll MOD = 1000000007;
const ll INF = 1000000000000000000L;
#ifdef __DEBUG

/**
 * For DEBUG
 * https://github.com/ta7uw/cpp-pyprint
 */
#include "cpp-pyprint/pyprint.h"

#endif

class mint {
public:
  long long x;

  constexpr mint(long long x = 0) : x((x % MOD + MOD) % MOD) {}

  constexpr mint operator-() const { return mint(-x); }

  constexpr mint &operator+=(const mint &rhs) {
    if ((x += rhs.x) >= MOD)
      x -= MOD;
    return *this;
  }

  constexpr mint &operator-=(const mint &rhs) {
    if ((x += MOD - rhs.x) >= MOD)
      x -= MOD;
    return *this;
  }

  constexpr mint &operator*=(const mint &rhs) {
    (x *= rhs.x) %= MOD;
    return *this;
  }

  constexpr mint operator+(const mint &rhs) const {
    mint res(*this);
    return res += rhs;
  }

  constexpr mint operator-(const mint &rhs) const {
    mint res(*this);
    return res -= rhs;
  }

  constexpr mint operator*(const mint &rhs) const {
    mint res(*this);
    return res *= rhs;
  }

  [[nodiscard]] constexpr mint pow(long long t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  [[nodiscard]] constexpr mint inv() const { return pow(MOD - 2); }

  mint &operator/=(mint rhs) { return *this *= rhs.inv(); }

  mint operator/(const mint rhs) const { return mint(*this) /= rhs; }

  constexpr bool operator==(const mint &rhs) const noexcept {
    return this->x == rhs.x;
  }

  constexpr bool operator!=(const mint &rhs) const noexcept {
    return this->x != rhs.x;
  }

  bool operator<(const mint &rhs) const noexcept { return this->x < rhs.x; }

  bool operator>(const mint &rhs) const noexcept { return this->x > rhs.x; }

  bool operator<=(const mint &rhs) const noexcept { return this->x <= rhs.x; }

  bool operator>=(const mint &rhs) const noexcept { return this->x >= rhs.x; }

  friend istream &operator>>(istream &is, mint &a) {
    long long t;
    is >> t;
    a = mint(t);
    return is;
  }

  friend ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
};

const int MAX = 2000000;
long long fac[MAX], finv[MAX], inv[MAX];
bool initialized = false;

/**
 * @fn
 * This function must be called at first.
 */
void combination_init() {
  initialized = true;
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int 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;
  }
}

/**
 * @fn
 * Calculates the combination  ( nCK % p: N choose K mod p).
 *
 * @brief constraint: 1 ≤ k ≤ n ≤ 10^7
 * @return nCK % MOD
 */
long long combination(int n, int k) {
  assert(initialized);
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

/**
 * @fn
 * Calculates the factorial of the number.
 *
 * @return N! % MOD
 */
long long factorial(int n) {
  assert(initialized);
  if (n < 0)
    return 0;
  return fac[n] % MOD;
}

/**
 * @fn
 * Calculates the combination ( nCK % p: N choose K mod p).
 *
 * @brief constraint: 1 ≤ k ≤ 10^7 and 1 ≤ n ≤ 10^9
 * @return nCK % MOD
 */
long long combination2(int n, int k) {
  assert(initialized);
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  ll res = 1;
  k = min(k, n - k);
  for (long long x = 1; x <= k; x++) {
    res *= (n - x + 1);
    res %= MOD;
    res *= inv[x];
    res %= MOD;
  }
  return res;
}

void solve() {
  ll K;
  string S;
  combination_init();
  cin >> K >> S;
  mint ans = 0;
  ll N = S.size();
  rep(i, K + 1) {
    ans += mint(25).pow(K - i) * combination(N + K - i - 1, N - 1) *
           mint(26).pow(i);
  }
  cout << ans << '\n';
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  solve();
  return 0;
}