Problem p02632 (C++)

Buggy submission (status: Runtime Error):

#pragma GCC optimize("O3")

#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include <prettyprint.hpp>
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]: ", d_err(__VA_ARGS__);
#else
#define debug(...) 83;
#endif

void d_err() { cerr << endl; }

template <typename H, typename... T> void d_err(H h, T... t) {
  cerr << h << " ";
  d_err(t...);
}

template <typename T> void print(T x) { cout << x << endl; }

#define ALL(x) (x).begin(), (x).end()
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define bcnt __builtin_popcountll

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<ll, ll> Pll;
typedef pair<int, int> Pin;

ll INF = 1e16;
int inf = 1e9;
ll MOD = 1e9 + 7;

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 {
    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;
  }
};

ostream &operator<<(ostream &os, const mint a) {
  os << a.x;

  return os;
}

////////////////////////////////

const int NMAX = int(1e6);
ll fact[NMAX + 1], finv[NMAX + 1], inv[NMAX + 1];

void comb_init(int N) {
  // init
  REP(i, 2) {
    fact[i] = 1;
    finv[i] = 1;
    inv[i] = 1;
  }

  FOR(i, 2, N + 1) {
    fact[i] = fact[i - 1] * i % MOD;
    inv[i] = MOD - MOD / i * inv[MOD % i] % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

mint comb(int n, int k) {
  if (n < k || n < 0 || k < 0)
    return 0;

  return mint(fact[n]) * finv[k] * finv[n - k];
}

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  int K;
  cin >> K;

  string S;
  cin >> S;

  comb_init(K + S.size() + 10);

  mint ans = 0;
  REP(k, K + 1) {
    mint c = comb(S.size() + k - 1, k);
    ans += c * mint(25).pow(k) * mint(26).pow(K - k);
  }

  print(ans);
}


Fixed submission:

#pragma GCC optimize("O3")

#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include <prettyprint.hpp>
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]: ", d_err(__VA_ARGS__);
#else
#define debug(...) 83;
#endif

void d_err() { cerr << endl; }

template <typename H, typename... T> void d_err(H h, T... t) {
  cerr << h << " ";
  d_err(t...);
}

template <typename T> void print(T x) { cout << x << endl; }

#define ALL(x) (x).begin(), (x).end()
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define bcnt __builtin_popcountll

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<ll, ll> Pll;
typedef pair<int, int> Pin;

ll INF = 1e16;
int inf = 1e9;
ll MOD = 1e9 + 7;

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 {
    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;
  }
};

ostream &operator<<(ostream &os, const mint a) {
  os << a.x;

  return os;
}

////////////////////////////////

const int NMAX = int(1e7);
ll fact[NMAX + 1], finv[NMAX + 1], inv[NMAX + 1];

void comb_init(int N) {
  // init
  REP(i, 2) {
    fact[i] = 1;
    finv[i] = 1;
    inv[i] = 1;
  }

  FOR(i, 2, N + 1) {
    fact[i] = fact[i - 1] * i % MOD;
    inv[i] = MOD - MOD / i * inv[MOD % i] % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

mint comb(int n, int k) {
  if (n < k || n < 0 || k < 0)
    return 0;

  return mint(fact[n]) * finv[k] * finv[n - k];
}

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  int K;
  cin >> K;

  string S;
  cin >> S;

  comb_init(K + S.size() + 10);

  mint ans = 0;
  REP(k, K + 1) {
    mint c = comb(S.size() + k - 1, k);
    ans += c * mint(25).pow(k) * mint(26).pow(K - k);
  }

  print(ans);
}