Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h> // ver2.7.1
#define int long long
#define endl "\n" // fflush(stdout);
#define ALL(v) (v).begin(), (v).end()
#define Vi vector<int>
#define VVi vector<Vi>
#define VVVi vector<VVi>
#define Vm vector<mint>
#define Vs vector<string>
#define Vd vector<double>
#define Vc vector<char>
#define Pii pair<int, int>
#define Pdd pair<double, double>
#define VPii vector<Pii>
#define Tiii tuple<int, int, int>
#define VTiii vector<Tiii>
#define PQi priority_queue<int>
#define PQir priority_queue<int, vector<int>, greater<int>>
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define itos to_string
#define stoi stoll
#define FI first
#define SE second
#define cYES cout << "YES" << endl
#define cNO cout << "NO" << endl
#define cYes cout << "Yes" << endl
#define cNo cout << "No" << endl
#define cyes cout << "yes" << endl
#define cno cout << "no" << endl
#define sortr(v) sort(v, greater<>())
#define rep(i, a, b) for (int i = a; i < b; i++)
#define repeq(i, a, b) for (int i = a; i <= b; i++)
#define repreq(i, a, b) for (int i = a; i >= b; i--)
#define dem(a, b) ((a + b - 1) / (b))
#define INF 3000000000000000000 // 3.0*10^18
#define MAX LLONG_MAX
#define PI acos(-1.0L)
// int MOD = 998244353; // case
const int MOD = 1000000007; // 10^9 + 7
const double EPS = 1e-10;

using namespace std;

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

/* debug */
template <typename T> // vector
ostream &operator<<(ostream &os, const vector<T> &V) {
  int N = V.size();
  if (N == 0) {
    os << '.';
    return os;
  }
  rep(i, 0, N - 1) { os << V[i] << ' '; }
  os << V[N - 1];
  return os;
}
template <typename T> // vector vector
ostream &operator<<(ostream &os, const vector<vector<T>> &V) {
  int N = V.size();
  rep(i, 0, N - 1) cout << V[i] << endl;
  cout << V[N - 1];
  return os;
}
template <typename T, typename S> // pair
ostream &operator<<(ostream &os, pair<T, S> const &P) {
  os << P.FI << ' ' << P.SE;
  return os;
}
template <typename T> // set
ostream &operator<<(ostream &os, set<T> &S) {
  auto it = S.begin();
  while (it != S.end()) {
    os << *it;
    if (next(it, 1) != S.end())
      os << ' ';
    it++;
  };
  return os;
}
template <typename T> // deque
ostream &operator<<(ostream &os, deque<T> &q) {
  for (auto it = q.begin(); it < q.end(); it++) {
    os << *it;
    if (it + 1 != q.end())
      os << " ";
  }
  return os;
}

/* useful */
template <typename T> void Vin(vector<T> &v) {
  int n = v.size();
  rep(i, 0, n) cin >> v[i];
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a / gcd(a, b) * b; }
int mypow(int x, int n, int m) {
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return mypow(x * x % m, n / 2, m);
  else
    return x * mypow(x, n - 1, m) % m;
}
int scomb(int n, int r) {
  if (r < 0 || r > n)
    return 0;
  if ((n - r) < r)
    r = n - r; // nCr
  int a = 1;
  for (int i = n; i > n - r; --i) {
    a = a * i;
  }
  for (int i = 1; i < r + 1; ++i) {
    a = a / i;
  }
  return a;
}
Vi vis(Vi &v) {
  Vi S(v.size() + 1);
  rep(i, 1, S.size()) S[i] += v[i - 1] + S[i - 1];
  return S;
}
int digitsum(int n) {
  int ret = 0;
  while (n > 0) {
    ret += n % 10;
    n /= 10;
  }
  return ret;
}
int digit(int k, int i) {
  string s = itos(k);
  return s[s.size() - i] - '0';
} // i桁目の数字
template <typename T> void press(T &v) {
  v.erase(unique(ALL(v)), v.end());
} // 圧縮
Vi zip(Vi b) {
  int Z = b.size(); // 座標圧縮
  Pii p[Z + 10];
  int a = b.size();
  Vi l(a);
  for (int i = 0; i < a; i++)
    p[i] = mp(b[i], i);
  sort(p, p + a);
  int w = 0;
  for (int i = 0; i < a; i++) {
    if (i && p[i].first != p[i - 1].first)
      w++;
    l[p[i].second] = w;
  }
  return l;
}
Vi nibe2V() {
  Vi a(63);
  int q = 1;
  rep(i, 0, 63) {
    a[i] = q;
    q *= 2;
  }
  return a;
}
Vi nibe = nibe2V(); // 2^n
int modiv(int a, int b) {
  return a * mypow(b, MOD - 2, MOD) % MOD;
} // a÷b(%MOD)
int SMALLER(Vi &a, int x) {
  return lower_bound(a.begin(), a.end(), x) - a.begin();
}
int orSMALLER(Vi &a, int x) {
  return upper_bound(a.begin(), a.end(), x) - a.begin();
}
int BIGGER(Vi &a, int x) { return a.size() - orSMALLER(a, x); }
int orBIGGER(Vi &a, int x) { return a.size() - SMALLER(a, x); }
int COUNT(Vi &a, int x) {
  return upper_bound(ALL(a), x) - lower_bound(ALL(a), x);
}
int maxind(Vi &a) { return max_element(ALL(a)) - a.begin(); }
int minind(Vi &a) { return min_element(ALL(a)) - a.begin(); }
Vi stpowV() {
  Vi a(300005);
  a[0] = 1;
  repeq(i, 1, 300004) a[i] = a[i - 1] * i % MOD;
  return a;
}
// Vi stpow = stpowV(); // 階乗配列(%MOD)

/****************************** START ******************************/

const int MAX_N = 200005;
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(MAX_N);

signed main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(12);
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = s.size();

  mint ans = 0;

  repeq(i, 0, k) {
    mint tmp = mypow(26, i, MOD);
    tmp *= mypow(25, k - i, MOD);
    tmp *= comb(n + k - i - 1, n - 1);
    ans += tmp;
  }

  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h> // ver2.7.1
#define int long long
#define endl "\n" // fflush(stdout);
#define ALL(v) (v).begin(), (v).end()
#define Vi vector<int>
#define VVi vector<Vi>
#define VVVi vector<VVi>
#define Vm vector<mint>
#define Vs vector<string>
#define Vd vector<double>
#define Vc vector<char>
#define Pii pair<int, int>
#define Pdd pair<double, double>
#define VPii vector<Pii>
#define Tiii tuple<int, int, int>
#define VTiii vector<Tiii>
#define PQi priority_queue<int>
#define PQir priority_queue<int, vector<int>, greater<int>>
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define itos to_string
#define stoi stoll
#define FI first
#define SE second
#define cYES cout << "YES" << endl
#define cNO cout << "NO" << endl
#define cYes cout << "Yes" << endl
#define cNo cout << "No" << endl
#define cyes cout << "yes" << endl
#define cno cout << "no" << endl
#define sortr(v) sort(v, greater<>())
#define rep(i, a, b) for (int i = a; i < b; i++)
#define repeq(i, a, b) for (int i = a; i <= b; i++)
#define repreq(i, a, b) for (int i = a; i >= b; i--)
#define dem(a, b) ((a + b - 1) / (b))
#define INF 3000000000000000000 // 3.0*10^18
#define MAX LLONG_MAX
#define PI acos(-1.0L)
// int MOD = 998244353; // case
const int MOD = 1000000007; // 10^9 + 7
const double EPS = 1e-10;

using namespace std;

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

/* debug */
template <typename T> // vector
ostream &operator<<(ostream &os, const vector<T> &V) {
  int N = V.size();
  if (N == 0) {
    os << '.';
    return os;
  }
  rep(i, 0, N - 1) { os << V[i] << ' '; }
  os << V[N - 1];
  return os;
}
template <typename T> // vector vector
ostream &operator<<(ostream &os, const vector<vector<T>> &V) {
  int N = V.size();
  rep(i, 0, N - 1) cout << V[i] << endl;
  cout << V[N - 1];
  return os;
}
template <typename T, typename S> // pair
ostream &operator<<(ostream &os, pair<T, S> const &P) {
  os << P.FI << ' ' << P.SE;
  return os;
}
template <typename T> // set
ostream &operator<<(ostream &os, set<T> &S) {
  auto it = S.begin();
  while (it != S.end()) {
    os << *it;
    if (next(it, 1) != S.end())
      os << ' ';
    it++;
  };
  return os;
}
template <typename T> // deque
ostream &operator<<(ostream &os, deque<T> &q) {
  for (auto it = q.begin(); it < q.end(); it++) {
    os << *it;
    if (it + 1 != q.end())
      os << " ";
  }
  return os;
}

/* useful */
template <typename T> void Vin(vector<T> &v) {
  int n = v.size();
  rep(i, 0, n) cin >> v[i];
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a / gcd(a, b) * b; }
int mypow(int x, int n, int m) {
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return mypow(x * x % m, n / 2, m);
  else
    return x * mypow(x, n - 1, m) % m;
}
int scomb(int n, int r) {
  if (r < 0 || r > n)
    return 0;
  if ((n - r) < r)
    r = n - r; // nCr
  int a = 1;
  for (int i = n; i > n - r; --i) {
    a = a * i;
  }
  for (int i = 1; i < r + 1; ++i) {
    a = a / i;
  }
  return a;
}
Vi vis(Vi &v) {
  Vi S(v.size() + 1);
  rep(i, 1, S.size()) S[i] += v[i - 1] + S[i - 1];
  return S;
}
int digitsum(int n) {
  int ret = 0;
  while (n > 0) {
    ret += n % 10;
    n /= 10;
  }
  return ret;
}
int digit(int k, int i) {
  string s = itos(k);
  return s[s.size() - i] - '0';
} // i桁目の数字
template <typename T> void press(T &v) {
  v.erase(unique(ALL(v)), v.end());
} // 圧縮
Vi zip(Vi b) {
  int Z = b.size(); // 座標圧縮
  Pii p[Z + 10];
  int a = b.size();
  Vi l(a);
  for (int i = 0; i < a; i++)
    p[i] = mp(b[i], i);
  sort(p, p + a);
  int w = 0;
  for (int i = 0; i < a; i++) {
    if (i && p[i].first != p[i - 1].first)
      w++;
    l[p[i].second] = w;
  }
  return l;
}
Vi nibe2V() {
  Vi a(63);
  int q = 1;
  rep(i, 0, 63) {
    a[i] = q;
    q *= 2;
  }
  return a;
}
Vi nibe = nibe2V(); // 2^n
int modiv(int a, int b) {
  return a * mypow(b, MOD - 2, MOD) % MOD;
} // a÷b(%MOD)
int SMALLER(Vi &a, int x) {
  return lower_bound(a.begin(), a.end(), x) - a.begin();
}
int orSMALLER(Vi &a, int x) {
  return upper_bound(a.begin(), a.end(), x) - a.begin();
}
int BIGGER(Vi &a, int x) { return a.size() - orSMALLER(a, x); }
int orBIGGER(Vi &a, int x) { return a.size() - SMALLER(a, x); }
int COUNT(Vi &a, int x) {
  return upper_bound(ALL(a), x) - lower_bound(ALL(a), x);
}
int maxind(Vi &a) { return max_element(ALL(a)) - a.begin(); }
int minind(Vi &a) { return min_element(ALL(a)) - a.begin(); }
Vi stpowV() {
  Vi a(300005);
  a[0] = 1;
  repeq(i, 1, 300004) a[i] = a[i - 1] * i % MOD;
  return a;
}
// Vi stpow = stpowV(); // 階乗配列(%MOD)

/****************************** START ******************************/

const int MAX_N = 2000005;
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(MAX_N);

signed main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(12);
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = s.size();

  mint ans = 0;

  repeq(i, 0, k) {
    mint tmp = mypow(26, i, MOD);
    tmp *= mypow(25, k - i, MOD);
    tmp *= comb(n + k - i - 1, n - 1);
    ans += tmp;
  }

  cout << ans << endl;

  return 0;
}