Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <utility>
#include <vector>

using namespace std;

/* template */

using ll = long long;

void debug_out() { std::cout << std::endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cout << H << " ";
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...)                                                             \
  cout << "debug: ";                                                           \
  debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif

#define rep(i, a, n) for (int i = (int)(a); i < (int)(n); i++)
#define rrep(i, a, n) for (int i = ((int)(n - 1)); i >= (int)(a); i--)
#define Rep(i, a, n) for (long long i = (long long)(a); i < (long long)(n); i++)
#define RRep(i, a, n)                                                          \
  for (long long i = ((long long)(n - 1ll)); i >= (long long)(a); i--)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()

template <typename T>
std::ostream &operator<<(std::ostream &os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}

struct Edge {
  int to;
  ll weight;
  Edge(int t, ll w) : to(t), weight(w) {}
};

struct edge {
  int from;
  int to;
  ll weight;
  edge(int f, int t, ll w) : from(f), to(t), weight(w) {}
};

using Graph = vector<vector<Edge>>;
using graph = vector<vector<int>>;
using edges = vector<edge>;

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;
}

template <class T> T lcm(T a, T b) { return (a * b) / gcd(a, b); }

ll ctoi(char c) {
  switch (c) {
  case '0':
    return 0;
  case '1':
    return 1;
  case '2':
    return 2;
  case '3':
    return 3;
  case '4':
    return 4;
  case '5':
    return 5;
  case '6':
    return 6;
  case '7':
    return 7;
  case '8':
    return 8;
  case '9':
    return 9;
  default:
    return 0;
  }
}

constexpr ll LNF = 1LL << 50;

constexpr int INF = 1e9 + 7;

const long double PI = 3.14159265358979323846;

vector<int> dx = {1, 0, -1, 1, -1, 0};
vector<int> dy = {1, 1, 1, 0, 0, -1};

/* template */

constexpr ll mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  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 { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= 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 { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

ll mod_pow(ll x, ll n, ll mod) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * x % mod;
    x = x * x % mod;
    n >>= 1;
  }
  return res;
}

// modintに依存
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];
  }
} comb(900005);

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  ll k;
  cin >> k;
  string s;
  cin >> s;
  ll n = s.size();
  mint ans = 0;
  Rep(i, 0, k + 1) {
    mint a = mod_pow(25, k - i, mod);
    a *= mod_pow(26, i, mod);
    a *= comb(n + k - i - 1, n - 1);
    ans += a;
  }
  cout << ans << endl;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <utility>
#include <vector>

using namespace std;

/* template */

using ll = long long;

void debug_out() { std::cout << std::endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cout << H << " ";
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...)                                                             \
  cout << "debug: ";                                                           \
  debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif

#define rep(i, a, n) for (int i = (int)(a); i < (int)(n); i++)
#define rrep(i, a, n) for (int i = ((int)(n - 1)); i >= (int)(a); i--)
#define Rep(i, a, n) for (long long i = (long long)(a); i < (long long)(n); i++)
#define RRep(i, a, n)                                                          \
  for (long long i = ((long long)(n - 1ll)); i >= (long long)(a); i--)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()

template <typename T>
std::ostream &operator<<(std::ostream &os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}

struct Edge {
  int to;
  ll weight;
  Edge(int t, ll w) : to(t), weight(w) {}
};

struct edge {
  int from;
  int to;
  ll weight;
  edge(int f, int t, ll w) : from(f), to(t), weight(w) {}
};

using Graph = vector<vector<Edge>>;
using graph = vector<vector<int>>;
using edges = vector<edge>;

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;
}

template <class T> T lcm(T a, T b) { return (a * b) / gcd(a, b); }

ll ctoi(char c) {
  switch (c) {
  case '0':
    return 0;
  case '1':
    return 1;
  case '2':
    return 2;
  case '3':
    return 3;
  case '4':
    return 4;
  case '5':
    return 5;
  case '6':
    return 6;
  case '7':
    return 7;
  case '8':
    return 8;
  case '9':
    return 9;
  default:
    return 0;
  }
}

constexpr ll LNF = 1LL << 50;

constexpr int INF = 1e9 + 7;

const long double PI = 3.14159265358979323846;

vector<int> dx = {1, 0, -1, 1, -1, 0};
vector<int> dy = {1, 1, 1, 0, 0, -1};

/* template */

constexpr ll mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  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 { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= 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 { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

ll mod_pow(ll x, ll n, ll mod) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * x % mod;
    x = x * x % mod;
    n >>= 1;
  }
  return res;
}

// modintに依存
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];
  }
} comb(5000005);

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  ll k;
  cin >> k;
  string s;
  cin >> s;
  ll n = s.size();
  mint ans = 0;
  Rep(i, 0, k + 1) {
    mint a = mod_pow(25, k - i, mod);
    a *= mod_pow(26, i, mod);
    a *= comb(n + k - i - 1, n - 1);
    ans += a;
  }
  cout << ans << endl;
}