Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define PI 3.14159265358979323846
#define MAXINF (1e18L)
#define INF (1e9L)
#define EPS (1e-9)
#define MOD ((ll)(1e9 + 7))
#define REP(i, n) for (int i = 0; i < int(n); ++i)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define RREP(i, n) for (int i = int(n) - 1; i >= 0; --i)
#define ALL(v) v.begin(), v.end()
#define FIND(v, x) (binary_search(ALL(v), (x)))
#define SORT(v) sort(ALL(v))
#define RSORT(v)                                                               \
  sort(ALL(v));                                                                \
  reverse(ALL(v))
#define DEBUG(x) cerr << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                                           \
  cerr << #v << ":";                                                           \
  for (int i = 0; i < v.size(); i++)                                           \
    cerr << " " << v[i];                                                       \
  cerr << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define pb push_back
#define fi first
#define se second
using namespace std;
template <class A> void pr(A a) { cout << (a) << endl; }
template <class A, class B> void pr(A a, B b) {
  cout << a << " ";
  pr(b);
}
template <class A, class B, class C> void pr(A a, B b, C c) {
  cout << a << " ";
  pr(b, c);
}
template <class A, class B, class C, class D> void pr(A a, B b, C c, D d) {
  cout << a << " ";
  pr(b, c, d);
}
template <class T> void pr_vec(vector<T> &v, char ep = ' ') {
  int n = v.size();
  REP(ni, n) {
    cout << v[ni];
    if (ni != n - 1)
      cout << ep;
  }
  cout << endl;
};
template <class T> inline bool chmin(T &a, T b) {
  return a > b ? a = b, true : false;
}
template <class T> inline bool chmax(T &a, T b) {
  return a < b ? a = b, true : false;
}
typedef long long ll;
typedef pair<int, int> pii;

typedef pair<ll, ll> pll;

template <uint32_t mod> struct ModInt {
  uint32_t a;
  ModInt &s(uint32_t vv) {
    a = vv < mod ? vv : vv - mod;
    return *this;
  }

  ModInt(int64_t x = 0) { s(x % mod + mod); }

  ModInt &operator+=(const ModInt &x) { return s(a + x.a); }
  ModInt &operator-=(const ModInt &x) { return s(a + mod - x.a); }
  ModInt &operator*=(const ModInt &x) {
    a = uint64_t(a) * x.a % mod;
    return *this;
  }
  ModInt &operator/=(const ModInt &x) {
    *this *= x.inv();
    return *this;
  }

  ModInt operator+(const ModInt &x) const { return ModInt(*this) += x; }
  ModInt operator-(const ModInt &x) const { return ModInt(*this) -= x; }
  ModInt operator*(const ModInt &x) const { return ModInt(*this) *= x; }
  ModInt operator/(const ModInt &x) const { return ModInt(*this) /= x; }
  bool operator==(const ModInt &x) const { return a == x.a; }
  bool operator!=(const ModInt &x) const { return a != x.a; }
  bool operator<(const ModInt &x) const { return a < x.a; }

  ModInt operator-() const { return ModInt() - *this; }
  ModInt pow(int64_t n) const {
    ModInt res(1), x(*this);
    while (n) {
      if (n & 1)
        res *= x;
      x *= x;
      n >>= 1;
    }
    return res;
  }

  ModInt inv() const { return pow(mod - 2); }
};

template <uint32_t mod> istream &operator>>(istream &in, ModInt<mod> &a) {
  return (in >> a.a);
}
template <uint32_t mod>
ostream &operator<<(ostream &out, const ModInt<mod> &a) {
  return (out << a.a);
}
using mint = ModInt<MOD>;

vector<mint> inv, fact, invfact;
void mod_build(int n = 200005) {
  fact.assign(n + 1, mint(1));
  inv.assign(n + 1, mint(1));
  invfact.assign(n + 1, mint(1));
  REP(i, n) {
    fact[i + 1] = fact[i] * mint(i + 1);
    if (i > 0)
      inv[i + 1] = mint() - inv[MOD % (i + 1)] * mint(MOD / (i + 1));
    invfact[i + 1] = invfact[i] * inv[i + 1];
  }
}
mint perm(int n, int k) {
  if (n < 0 || k < 0 || k > n)
    return 0;
  return fact[n] * invfact[n - k];
}
mint comb(int n, int k) {
  if (n < 0 || k < 0 || k > n)
    return 0;
  return (fact[n] * invfact[n - k]) * invfact[k];
}
// O(n)が間に合わなさそうなnに対応したcomb
// フェルマーの小定理と逆元を用いて計算
mint comb2(ll n, int k) {
  mint X(n), Y(1);
  REP(i, k - 1) {
    X *= n - i - 1;
    Y *= i + 2;
  }
  return X * Y.pow(MOD - 2);
}

int main(void) {
  int K;
  cin >> K;
  string S;
  cin >> S;
  int n = S.size();

  mod_build();
  mint ans = 0;
  Rep(i, 0, K + 1) {
    ans += comb(n + K - i - 1, n - 1) * mint(26).pow(i) * mint(25).pow(K - i);
  }
  pr(ans);
}

Fixed submission:
#include <bits/stdc++.h>
#define PI 3.14159265358979323846
#define MAXINF (1e18L)
#define INF (1e9L)
#define EPS (1e-9)
#define MOD ((ll)(1e9 + 7))
#define REP(i, n) for (int i = 0; i < int(n); ++i)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define RREP(i, n) for (int i = int(n) - 1; i >= 0; --i)
#define ALL(v) v.begin(), v.end()
#define FIND(v, x) (binary_search(ALL(v), (x)))
#define SORT(v) sort(ALL(v))
#define RSORT(v)                                                               \
  sort(ALL(v));                                                                \
  reverse(ALL(v))
#define DEBUG(x) cerr << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                                           \
  cerr << #v << ":";                                                           \
  for (int i = 0; i < v.size(); i++)                                           \
    cerr << " " << v[i];                                                       \
  cerr << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define pb push_back
#define fi first
#define se second
using namespace std;
template <class A> void pr(A a) { cout << (a) << endl; }
template <class A, class B> void pr(A a, B b) {
  cout << a << " ";
  pr(b);
}
template <class A, class B, class C> void pr(A a, B b, C c) {
  cout << a << " ";
  pr(b, c);
}
template <class A, class B, class C, class D> void pr(A a, B b, C c, D d) {
  cout << a << " ";
  pr(b, c, d);
}
template <class T> void pr_vec(vector<T> &v, char ep = ' ') {
  int n = v.size();
  REP(ni, n) {
    cout << v[ni];
    if (ni != n - 1)
      cout << ep;
  }
  cout << endl;
};
template <class T> inline bool chmin(T &a, T b) {
  return a > b ? a = b, true : false;
}
template <class T> inline bool chmax(T &a, T b) {
  return a < b ? a = b, true : false;
}
typedef long long ll;
typedef pair<int, int> pii;

typedef pair<ll, ll> pll;

template <uint32_t mod> struct ModInt {
  uint32_t a;
  ModInt &s(uint32_t vv) {
    a = vv < mod ? vv : vv - mod;
    return *this;
  }

  ModInt(int64_t x = 0) { s(x % mod + mod); }

  ModInt &operator+=(const ModInt &x) { return s(a + x.a); }
  ModInt &operator-=(const ModInt &x) { return s(a + mod - x.a); }
  ModInt &operator*=(const ModInt &x) {
    a = uint64_t(a) * x.a % mod;
    return *this;
  }
  ModInt &operator/=(const ModInt &x) {
    *this *= x.inv();
    return *this;
  }

  ModInt operator+(const ModInt &x) const { return ModInt(*this) += x; }
  ModInt operator-(const ModInt &x) const { return ModInt(*this) -= x; }
  ModInt operator*(const ModInt &x) const { return ModInt(*this) *= x; }
  ModInt operator/(const ModInt &x) const { return ModInt(*this) /= x; }
  bool operator==(const ModInt &x) const { return a == x.a; }
  bool operator!=(const ModInt &x) const { return a != x.a; }
  bool operator<(const ModInt &x) const { return a < x.a; }

  ModInt operator-() const { return ModInt() - *this; }
  ModInt pow(int64_t n) const {
    ModInt res(1), x(*this);
    while (n) {
      if (n & 1)
        res *= x;
      x *= x;
      n >>= 1;
    }
    return res;
  }

  ModInt inv() const { return pow(mod - 2); }
};

template <uint32_t mod> istream &operator>>(istream &in, ModInt<mod> &a) {
  return (in >> a.a);
}
template <uint32_t mod>
ostream &operator<<(ostream &out, const ModInt<mod> &a) {
  return (out << a.a);
}
using mint = ModInt<MOD>;

vector<mint> inv, fact, invfact;
void mod_build(int n = 2000010) {
  fact.assign(n + 1, mint(1));
  inv.assign(n + 1, mint(1));
  invfact.assign(n + 1, mint(1));
  REP(i, n) {
    fact[i + 1] = fact[i] * mint(i + 1);
    if (i > 0)
      inv[i + 1] = mint() - inv[MOD % (i + 1)] * mint(MOD / (i + 1));
    invfact[i + 1] = invfact[i] * inv[i + 1];
  }
}
mint perm(int n, int k) {
  if (n < 0 || k < 0 || k > n)
    return 0;
  return fact[n] * invfact[n - k];
}
mint comb(int n, int k) {
  if (n < 0 || k < 0 || k > n)
    return 0;
  return (fact[n] * invfact[n - k]) * invfact[k];
}
// O(n)が間に合わなさそうなnに対応したcomb
// フェルマーの小定理と逆元を用いて計算
mint comb2(ll n, int k) {
  mint X(n), Y(1);
  REP(i, k - 1) {
    X *= n - i - 1;
    Y *= i + 2;
  }
  return X * Y.pow(MOD - 2);
}

int main(void) {
  int K;
  cin >> K;
  string S;
  cin >> S;
  int n = S.size();

  mod_build();
  mint ans = 0;
  Rep(i, 0, K + 1) {
    ans += comb(n + K - i - 1, n - 1) * mint(26).pow(i) * mint(25).pow(K - i);
  }
  pr(ans);
}