Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define Per(i, sta, n) for (int i = n - 1; i >= sta; i--)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef long double ld;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

template <int mod> struct ModInt {
  long long x;

  ModInt() : x(0) {}
  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  explicit operator int() const { return x; }

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    return ModInt(u);
  }

  ModInt power(long long p) const {
    int a = x;
    if (p == 0)
      return 1;
    if (p == 1)
      return ModInt(a);
    if (p % 2 == 1)
      return (ModInt(a) * ModInt(a)).power(p / 2) * ModInt(a);
    else
      return (ModInt(a) * ModInt(a)).power(p / 2);
  }

  ModInt power(const ModInt p) const { return ((ModInt)x).power(p.x); }

  friend ostream &operator<<(ostream &os, const ModInt<mod> &p) {
    return os << p.x;
  }
  friend istream &operator>>(istream &is, ModInt<mod> &a) {
    long long x;
    is >> x;
    a = ModInt<mod>(x);
    return (is);
  }
};

using modint = ModInt<mod>;

struct ModFac {
public:
  vector<modint> f, i_f;
  int n;

  ModFac(int n_) {
    n = n_;
    f.resize(n + 1, 1);
    i_f.resize(n + 1, 1);
    for (int i = 0; i < n; i++) {
      f[i + 1] = f[i] * (modint)(i + 1);
    }
    i_f[n] = f[n].power(mod - 2);
    for (int i = n - 1; i >= 0; i--) {
      i_f[i] = i_f[i + 1] * (modint)(i + 1);
    }
  }
  ModFac(modint n_) {
    n = (int)n_;
    f.resize(n + 1, 1);
    i_f.resize(n + 1, 1);
    for (int i = 0; i < n; i++) {
      f[i + 1] = f[i] * (modint)(i + 1);
    }
    i_f[n] = f[n].power(mod - 2);
    for (int i = n - 1; i >= 0; i--) {
      i_f[i] = i_f[i + 1] * (modint)(i + 1);
    }
  }

  modint factorial(int x) {
    // cout << f.size() << endl;
    return f[x];
  }

  modint inv_factorial(int x) { return i_f[x]; }

  modint comb(int m, int k) {
    if (m < 0 or k < 0)
      return 0;
    if (m < k)
      return 0;
    return f[m] * i_f[k] * i_f[m - k];
  }
};

int n, k;
ModFac MF(200010);
modint ans[200010];

void solve() {
  cin >> k;
  string s;
  cin >> s;
  n = s.length();
  rep(i, n + k + 1) {
    ans[i] = MF.comb(n + k, i) * ((modint)25).power(n + k - i);
  }
  modint A = 0;
  Rep(i, n, n + k + 1) { A += ans[i]; }
  cout << A << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define Per(i, sta, n) for (int i = n - 1; i >= sta; i--)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef long double ld;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

template <int mod> struct ModInt {
  long long x;

  ModInt() : x(0) {}
  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  explicit operator int() const { return x; }

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    return ModInt(u);
  }

  ModInt power(long long p) const {
    int a = x;
    if (p == 0)
      return 1;
    if (p == 1)
      return ModInt(a);
    if (p % 2 == 1)
      return (ModInt(a) * ModInt(a)).power(p / 2) * ModInt(a);
    else
      return (ModInt(a) * ModInt(a)).power(p / 2);
  }

  ModInt power(const ModInt p) const { return ((ModInt)x).power(p.x); }

  friend ostream &operator<<(ostream &os, const ModInt<mod> &p) {
    return os << p.x;
  }
  friend istream &operator>>(istream &is, ModInt<mod> &a) {
    long long x;
    is >> x;
    a = ModInt<mod>(x);
    return (is);
  }
};

using modint = ModInt<mod>;

struct ModFac {
public:
  vector<modint> f, i_f;
  int n;

  ModFac(int n_) {
    n = n_;
    f.resize(n + 1, 1);
    i_f.resize(n + 1, 1);
    for (int i = 0; i < n; i++) {
      f[i + 1] = f[i] * (modint)(i + 1);
    }
    i_f[n] = f[n].power(mod - 2);
    for (int i = n - 1; i >= 0; i--) {
      i_f[i] = i_f[i + 1] * (modint)(i + 1);
    }
  }
  ModFac(modint n_) {
    n = (int)n_;
    f.resize(n + 1, 1);
    i_f.resize(n + 1, 1);
    for (int i = 0; i < n; i++) {
      f[i + 1] = f[i] * (modint)(i + 1);
    }
    i_f[n] = f[n].power(mod - 2);
    for (int i = n - 1; i >= 0; i--) {
      i_f[i] = i_f[i + 1] * (modint)(i + 1);
    }
  }

  modint factorial(int x) {
    // cout << f.size() << endl;
    return f[x];
  }

  modint inv_factorial(int x) { return i_f[x]; }

  modint comb(int m, int k) {
    if (m < 0 or k < 0)
      return 0;
    if (m < k)
      return 0;
    return f[m] * i_f[k] * i_f[m - k];
  }
};

int n, k;
ModFac MF(2000010);
modint ans[2000010];

void solve() {
  cin >> k;
  string s;
  cin >> s;
  n = s.length();
  rep(i, n + k + 1) {
    ans[i] = MF.comb(n + k, i) * ((modint)25).power(n + k - i);
  }
  modint A = 0;
  Rep(i, n, n + k + 1) { A += ans[i]; }
  cout << A << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}