Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define int long long
#define dub long double
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define ROF(i, a, b) for (int i = (a); i > (b); --i)
#define CASET                                                                  \
  int _T;                                                                      \
  cin >> _T;                                                                   \
  FOR(caset, 1, _T + 1)
#define pii pair<int, int>
#define t3i tuple<int, int, int>
#define mii map<int, int>
#define vi vector<int>
#define vb vector<bool>
#define vs vector<string>
#define vvi vector<vi>
#define vvb vector<vb>
#define vpii vector<pii>
#define vt3i vector<t3i>
#define vvpii vector<vpii>
#define sq(x) (x) * (x)
#define inv(x) powa(x, MOD - 2)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define SIZE(x) (signed)(x).size()
#define SUM(x) accumulate(ALL(x), 0LL)

long long INF = 1LL << 60;
long long MOD = 1e9 + 7;

namespace output {
void pr(short x) { cout << x; }
void pr(signed x) { cout << x; }
void pr(long x) { cout << x; }
void pr(long long x) { cout << x; }
void pr(unsigned short x) { cout << x; }
void pr(unsigned x) { cout << x; }
void pr(unsigned long x) { cout << x; }
void pr(unsigned long long x) { cout << x; }
void pr(float x) { cout << x; }
void pr(double x) { cout << x; }
void pr(long double x) { cout << x; }
void pr(char x) { cout << x; }
void pr(const char *x) { cout << x; }
void pr(const string &x) { cout << x; }
void pr(bool x) { cout << x; }
template <size_t sz> void pr(const bitset<sz> &x) { cout << x; }
template <class T> void pr(const complex<T> &x) { cout << x; }

template <class T1, class T2> void pr(const pair<T1, T2> &x);
template <class T1, class T2, class T3> void pr(const tuple<T1, T2, T3> &x);
template <class T> void pr(const T &x);

template <class T, class... Ts> void pr(const T &t, const Ts &...ts) {
  pr(t);
  pr(ts...);
}
template <class T1, class T2> void pr(const pair<T1, T2> &x) {
  pr("(", x.first, ", ", x.second, ")");
}
template <class T1, class T2, class T3> void pr(const tuple<T1, T2, T3> &x) {
  pr("(", get<0>(x), ", ", get<1>(x), ", ", get<2>(x), ")");
}
template <class T> void pr(const T &x) {
  pr("["); // const iterator needed for vector<bool>
  bool fst = 1;
  for (const auto &a : x)
    pr(!fst ? ", " : "", a), fst = 0;
  pr("]");
}

void print() { pr("\n"); } // print w/ spaces
template <class T, class... Ts> void print(const T &t, const Ts &...ts) {
  pr(t);
  if (sizeof...(ts))
    pr(" ");
  print(ts...);
}
} // namespace output
using namespace output;

template <typename T = int> vector<T> create(size_t n) { return vector<T>(n); }
template <typename T, typename... Args> auto create(size_t n, Args... args) {
  return vector<decltype(create<T>(args...))>(n, create<T>(args...));
}

template <class T1, class T2>
istream &operator>>(istream &is, pair<T1, T2> &x) {
  is >> x.first >> x.second;
  return is;
}
template <class T1, class T2, class T3>
istream &operator>>(istream &is, tuple<T1, T2, T3> &x) {
  is >> get<0>(x) >> get<1>(x) >> get<2>(x);
  return is;
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
int powa(int base, int exp) {
  int res = 1;
  while (exp) {
    if (exp & 1)
      res = res * base % MOD;
    base = base * base % MOD;
    exp >>= 1;
  }
  return res;
}
bool mini(int &a, int b) { return b < a ? a = b, 1 : 0; }
bool maxi(int &a, int b) { return b > a ? a = b, 1 : 0; }

signed main() {
  ios::sync_with_stdio(0), cin.tie(0);

  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = SIZE(s);

  const int BIG = 2e5 + 5;
  vi fact(BIG);
  fact[0] = 1;
  FOR(i, 1, BIG) fact[i] = i * fact[i - 1] % MOD;
  vi invfact(BIG);
  invfact.back() = inv(fact.back());
  ROF(i, BIG - 2, -1) invfact[i] = (i + 1) * invfact[i + 1] % MOD;

  auto nCr = [&](int n, int r) {
    if (r < 0 || r > n)
      return 0LL;
    return fact[n] * invfact[r] % MOD * invfact[n - r] % MOD;
  };

  int res = 0;
  FOR(h, n, n + k + 1) {
    int cur =
        nCr(h - 1, n - 1) * powa(25, h - n) % MOD * powa(26, n + k - h) % MOD;
    res = (res + cur) % MOD;
  }
  print(res);
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define int long long
#define dub long double
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define ROF(i, a, b) for (int i = (a); i > (b); --i)
#define CASET                                                                  \
  int _T;                                                                      \
  cin >> _T;                                                                   \
  FOR(caset, 1, _T + 1)
#define pii pair<int, int>
#define t3i tuple<int, int, int>
#define mii map<int, int>
#define vi vector<int>
#define vb vector<bool>
#define vs vector<string>
#define vvi vector<vi>
#define vvb vector<vb>
#define vpii vector<pii>
#define vt3i vector<t3i>
#define vvpii vector<vpii>
#define sq(x) (x) * (x)
#define inv(x) powa(x, MOD - 2)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define SIZE(x) (signed)(x).size()
#define SUM(x) accumulate(ALL(x), 0LL)

long long INF = 1LL << 60;
long long MOD = 1e9 + 7;

namespace output {
void pr(short x) { cout << x; }
void pr(signed x) { cout << x; }
void pr(long x) { cout << x; }
void pr(long long x) { cout << x; }
void pr(unsigned short x) { cout << x; }
void pr(unsigned x) { cout << x; }
void pr(unsigned long x) { cout << x; }
void pr(unsigned long long x) { cout << x; }
void pr(float x) { cout << x; }
void pr(double x) { cout << x; }
void pr(long double x) { cout << x; }
void pr(char x) { cout << x; }
void pr(const char *x) { cout << x; }
void pr(const string &x) { cout << x; }
void pr(bool x) { cout << x; }
template <size_t sz> void pr(const bitset<sz> &x) { cout << x; }
template <class T> void pr(const complex<T> &x) { cout << x; }

template <class T1, class T2> void pr(const pair<T1, T2> &x);
template <class T1, class T2, class T3> void pr(const tuple<T1, T2, T3> &x);
template <class T> void pr(const T &x);

template <class T, class... Ts> void pr(const T &t, const Ts &...ts) {
  pr(t);
  pr(ts...);
}
template <class T1, class T2> void pr(const pair<T1, T2> &x) {
  pr("(", x.first, ", ", x.second, ")");
}
template <class T1, class T2, class T3> void pr(const tuple<T1, T2, T3> &x) {
  pr("(", get<0>(x), ", ", get<1>(x), ", ", get<2>(x), ")");
}
template <class T> void pr(const T &x) {
  pr("["); // const iterator needed for vector<bool>
  bool fst = 1;
  for (const auto &a : x)
    pr(!fst ? ", " : "", a), fst = 0;
  pr("]");
}

void print() { pr("\n"); } // print w/ spaces
template <class T, class... Ts> void print(const T &t, const Ts &...ts) {
  pr(t);
  if (sizeof...(ts))
    pr(" ");
  print(ts...);
}
} // namespace output
using namespace output;

template <typename T = int> vector<T> create(size_t n) { return vector<T>(n); }
template <typename T, typename... Args> auto create(size_t n, Args... args) {
  return vector<decltype(create<T>(args...))>(n, create<T>(args...));
}

template <class T1, class T2>
istream &operator>>(istream &is, pair<T1, T2> &x) {
  is >> x.first >> x.second;
  return is;
}
template <class T1, class T2, class T3>
istream &operator>>(istream &is, tuple<T1, T2, T3> &x) {
  is >> get<0>(x) >> get<1>(x) >> get<2>(x);
  return is;
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
int powa(int base, int exp) {
  int res = 1;
  while (exp) {
    if (exp & 1)
      res = res * base % MOD;
    base = base * base % MOD;
    exp >>= 1;
  }
  return res;
}
bool mini(int &a, int b) { return b < a ? a = b, 1 : 0; }
bool maxi(int &a, int b) { return b > a ? a = b, 1 : 0; }

signed main() {
  ios::sync_with_stdio(0), cin.tie(0);

  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = SIZE(s);

  const int BIG = 2e6 + 5;
  vi fact(BIG);
  fact[0] = 1;
  FOR(i, 1, BIG) fact[i] = i * fact[i - 1] % MOD;
  vi invfact(BIG);
  invfact.back() = inv(fact.back());
  ROF(i, BIG - 2, -1) invfact[i] = (i + 1) * invfact[i + 1] % MOD;

  auto nCr = [&](int n, int r) {
    if (r < 0 || r > n)
      return 0LL;
    return fact[n] * invfact[r] % MOD * invfact[n - r] % MOD;
  };

  int res = 0;
  FOR(h, n, n + k + 1) {
    int cur =
        nCr(h - 1, n - 1) * powa(25, h - n) % MOD * powa(26, n + k - h) % MOD;
    res = (res + cur) % MOD;
  }
  print(res);
}