Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;

using namespace std;

const double PI = 3.14159265358979323846;
typedef long long ll;
const double EPS = 1e-9;
#define rep(i, n) for (int i = 0; i < (n); ++i)
// #define rep(i, n) for (ll i = 0; i < (n); ++i)
typedef pair<ll, ll> P;
const ll INF = 10e17;
#define cmin(x, y) x = min(x, y)
#define cmax(x, y) x = max(x, y)
#define ret() return 0;

double equal(double a, double b) { return fabs(a - b) < DBL_EPSILON; }

std::istream &operator>>(std::istream &in, set<int> &o) {
  int a;
  in >> a;
  o.insert(a);
  return in;
}

std::istream &operator>>(std::istream &in, queue<int> &o) {
  ll a;
  in >> a;
  o.push(a);
  return in;
}

bool contain(set<int> &s, int a) { return s.find(a) != s.end(); }

// ofstream outfile("log.txt");
// outfile << setw(6) << setfill('0') << prefecture << setw(6) << setfill('0')
// << rank << endl;
//  std::cout << std::bitset<8>(9);
// const ll mod = 1e10;

typedef priority_queue<ll, vector<ll>, greater<ll>> PQ_ASK;

const int mod = 1000000007;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}

  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }

  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }

  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }

  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }

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

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

  friend std::istream &operator>>(std::istream &in, mint &o) {
    ll a;
    in >> a;
    o = a;
    return in;
  }

  friend std::ostream &operator<<(std::ostream &out, const mint &o) {
    out << o.x;
    return out;
  }
};

struct combination {
  vector<mint> fact, ifact;

  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }

  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
} combination(1000000);

int main() {

  int k;
  string s;
  cin >> k >> s;

  int n = s.size();
  mint ans = 0;

  rep(i, k + 1) {
    ans += combination(k - i + n - 1, n - 1) * mint(25).pow(k - i) *
           mint(26).pow(i);
  }
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;

using namespace std;

const double PI = 3.14159265358979323846;
typedef long long ll;
const double EPS = 1e-9;
#define rep(i, n) for (int i = 0; i < (n); ++i)
// #define rep(i, n) for (ll i = 0; i < (n); ++i)
typedef pair<ll, ll> P;
const ll INF = 10e17;
#define cmin(x, y) x = min(x, y)
#define cmax(x, y) x = max(x, y)
#define ret() return 0;

double equal(double a, double b) { return fabs(a - b) < DBL_EPSILON; }

std::istream &operator>>(std::istream &in, set<int> &o) {
  int a;
  in >> a;
  o.insert(a);
  return in;
}

std::istream &operator>>(std::istream &in, queue<int> &o) {
  ll a;
  in >> a;
  o.push(a);
  return in;
}

bool contain(set<int> &s, int a) { return s.find(a) != s.end(); }

// ofstream outfile("log.txt");
// outfile << setw(6) << setfill('0') << prefecture << setw(6) << setfill('0')
// << rank << endl;
//  std::cout << std::bitset<8>(9);
// const ll mod = 1e10;

typedef priority_queue<ll, vector<ll>, greater<ll>> PQ_ASK;

const int mod = 1000000007;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}

  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }

  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }

  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }

  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }

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

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

  friend std::istream &operator>>(std::istream &in, mint &o) {
    ll a;
    in >> a;
    o = a;
    return in;
  }

  friend std::ostream &operator<<(std::ostream &out, const mint &o) {
    out << o.x;
    return out;
  }
};

struct combination {
  vector<mint> fact, ifact;

  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }

  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
} combination(10000000);

int main() {

  int k;
  string s;
  cin >> k >> s;

  int n = s.size();
  mint ans = 0;

  rep(i, k + 1) {
    ans += combination(k - i + n - 1, n - 1) * mint(25).pow(k - i) *
           mint(26).pow(i);
  }
  cout << ans << endl;
}