Problem p02632 (C++)

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

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define MP make_pair
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
  cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
  int x12345;                                                                  \
  cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
const long long MOD = 1e9 + 7;
///////////////////////////////////////////////
// for template
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|

template <int MOD> struct Fp {
  long long val;

  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += 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 = val % MOD;
    return *this;
  }

  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD * ((-val + MOD - 1) / MOD);
    val = 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 istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> 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;
  }
};

using mint = Fp<MOD>;

const int NMAX = 1010000;
ll fac[NMAX], finv[NMAX], inv[NMAX];
void combinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;

  for (int i = 2; i <= NMAX - 1; ++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;
  }
}

long long comb(int n, int r) {
  if (n < r)
    return 0;
  if (n < 0 || r < 0)
    return 0;
  return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
}

int main() {
  combinit();
  ll K;
  cin >> K;
  string S;
  cin >> S;
  ll N = S.length();
  mint ans = 0;
  FORq(l, N, N + K) {
    ans += (mint)comb(l - 1, N - 1) * modpow((mint)25, l - N) *
           modpow((mint)26, N + K - l);
  }
  cout << ans << endl;
}

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

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define MP make_pair
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
  cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
  int x12345;                                                                  \
  cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
const long long MOD = 1e9 + 7;
///////////////////////////////////////////////
// for template
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|

template <int MOD> struct Fp {
  long long val;

  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += 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 = val % MOD;
    return *this;
  }

  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD * ((-val + MOD - 1) / MOD);
    val = 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 istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> 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;
  }
};

using mint = Fp<MOD>;

const int NMAX = 3010000;
ll fac[NMAX], finv[NMAX], inv[NMAX];
void combinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;

  for (int i = 2; i <= NMAX - 1; ++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;
  }
}

long long comb(int n, int r) {
  if (n < r)
    return 0;
  if (n < 0 || r < 0)
    return 0;
  return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
}

int main() {
  combinit();
  ll K;
  cin >> K;
  string S;
  cin >> S;
  ll N = S.length();
  mint ans = 0;
  FORq(l, N, N + K) {
    ans += (mint)comb(l - 1, N - 1) * modpow((mint)25, l - N) *
           modpow((mint)26, N + K - l);
  }
  cout << ans << endl;
}