Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#pragma region
#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;
#define endl "\n"
#define MP make_pair
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORR(x, arr) for (auto &x : arr)
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define VPII vector<PII>
#define VPLL vector<PLL>
#define FI first
#define SE second
#define ALL(x) (x).begin(), (x).end()
constexpr int INF = 1 << 30;
constexpr ll LINF = 1LL << 60;
constexpr ll mod = 1e9 + 7;
constexpr int NIL = -1;
template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
#pragma endregion
//-------------------

struct mint {
  ll x;
  mint(ll x = 0) : x(x % mod) {}

  bool operator==(const mint a) const { return x == a.x; }
  bool operator!=(const mint a) const { return x != a.x; }
  bool operator>=(const mint a) { return (x >= a.x) ? 1 : 0; }
  bool operator<(const mint a) { return !(*this >= a); }
  bool operator>(const mint a) { return (x > a.x) ? 1 : 0; }
  bool operator<=(const mint a) { return !(*this > a); }
  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; // 2 square
    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;
  }
};

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    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];
  }
} comb(100005);

mint f(int n, int k) {
  if (n < 0)
    return 0;
  // nPk = nCk * k!
  mint res = comb(n, k);
  res *= comb.fact[k];
  return res;
}

int n;
string s;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false); // cout << fixed << setprecision(15);
  cin >> n >> s;
  mint ans(0);
  int sz = n + s.size();
  ans += mint(26).pow(sz);

  FOR(i, 0, s.size() - 1) { ans -= comb(sz, i) * (mint(25).pow(sz - i)); }

  cout << ans.x << endl;
  return 0;
}


Fixed submission:
#pragma region
#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;
#define endl "\n"
#define MP make_pair
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORR(x, arr) for (auto &x : arr)
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define VPII vector<PII>
#define VPLL vector<PLL>
#define FI first
#define SE second
#define ALL(x) (x).begin(), (x).end()
constexpr int INF = 1 << 30;
constexpr ll LINF = 1LL << 60;
constexpr ll mod = 1e9 + 7;
constexpr int NIL = -1;
template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
#pragma endregion
//-------------------

struct mint {
  ll x;
  mint(ll x = 0) : x(x % mod) {}

  bool operator==(const mint a) const { return x == a.x; }
  bool operator!=(const mint a) const { return x != a.x; }
  bool operator>=(const mint a) { return (x >= a.x) ? 1 : 0; }
  bool operator<(const mint a) { return !(*this >= a); }
  bool operator>(const mint a) { return (x > a.x) ? 1 : 0; }
  bool operator<=(const mint a) { return !(*this > a); }
  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; // 2 square
    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;
  }
};

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    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];
  }
} comb(2000005);

mint f(int n, int k) {
  if (n < 0)
    return 0;
  // nPk = nCk * k!
  mint res = comb(n, k);
  res *= comb.fact[k];
  return res;
}

int n;
string s;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false); // cout << fixed << setprecision(15);
  cin >> n >> s;
  mint ans(0);
  int sz = n + s.size();
  ans += mint(26).pow(sz);

  FOR(i, 0, s.size() - 1) { ans -= comb(sz, i) * (mint(25).pow(sz - i)); }

  cout << ans.x << endl;
  return 0;
}