Problem p02769 (C++)

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

#define int long long
typedef pair<int, int> P;
#define mod 1000000007
#define INF (1LL << 60)

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define rep1(i, a, n) for (int i = (a), i##_len = (n); i < i##_len; ++i)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define YES puts("YES")
#define Yes puts("Yes")
#define NO puts("NO")
#define No puts("No")
#define ALL(v) (v).begin(), (v).end()
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }

const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

#if 1
//-------------
// DUMPマクロ
// https://www.creativ.xyz/dump-cpp-652/

// vector
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
  os << "(" << pair_var.first << ", " << pair_var.second << ")";
  return os;
}
// vector
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
  }
  os << "}";
  return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << *itr;
    itr++;
    if (itr != map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
// set
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) {
  os << "{";
  repi(itr, set_var) {
    os << *itr;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}

#define DUMPOUT cerr

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  DUMPOUT << head;
  if (sizeof...(Tail) > 0) {
    DUMPOUT << ", ";
  }
  dump_func(std::move(tail)...);
}
#ifdef DEBUG
#define DEB
#define dump(...)                                                              \
  DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                              \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "    ",                                                           \
      dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

#endif

int modpow(int a, int k, int p) {
  int ans = 1;
  while (k > 0) {
    if (k % 2 == 1) {
      ans *= a;
      ans %= p;
    }
    a = (a * a) % p;
    k = k / 2;
  }
  return ans;
}

// ax+by=1の解
int extgcd(int a, int b, int &x, int &y) {
  int d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}

// mod mでのaの逆数
int mod_inv(int a, int m) {
  int x, y;
  extgcd(a, m, x, y);
  return (m + x % m) % m;
}

// nの階乗 mod p
int factorial(int n, int p) {
  int r = 1;
  for (int i = 1; i <= n; i++)
    r = (r * i) % p;
  return r;
}

// nPr mod p
int permutation(int n, int r, int p) {
  int a = 1;
  for (int i = n - r + 1; i <= n; i++)
    a = (a * i) % p;
  return a;
}

// 階乗の事前計算
const int MAX = 223456;
int fac[MAX], fac_inv[MAX];

void make_table(int N) {
  fac[0] = fac[1] = 1;
  for (int i = 2; i <= N; i++)
    fac[i] = (fac[i - 1] * i) % mod;
  for (int i = 0; i <= N; i++)
    fac_inv[i] = mod_inv(fac[i], mod);
}

int combination(int n, int r, int p) {
  return (fac[n] * fac_inv[r]) % p * fac_inv[n - r] % p;
}

void solve() {
  int n, k;
  cin >> n >> k;

  make_table(523456);

  if (k >= n) {
    cout << combination(2 * n - 1, n, mod) << endl;
    return;
  }

  int ans = 0;
  // zeroの数を固定
  for (int x = 0; x <= k; x++) {
    ans += combination(n, x, mod) * combination(n - 1, n - x - 1, mod) % mod;
    ans %= mod;
  }
  cout << ans << endl;
}

signed main() {
  cout << fixed << setprecision(18);
  cerr << fixed << setprecision(18);
  solve();
}


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

#define int long long
typedef pair<int, int> P;
#define mod 1000000007
#define INF (1LL << 60)

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define rep1(i, a, n) for (int i = (a), i##_len = (n); i < i##_len; ++i)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define YES puts("YES")
#define Yes puts("Yes")
#define NO puts("NO")
#define No puts("No")
#define ALL(v) (v).begin(), (v).end()
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }

const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

#if 1
//-------------
// DUMPマクロ
// https://www.creativ.xyz/dump-cpp-652/

// vector
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
  os << "(" << pair_var.first << ", " << pair_var.second << ")";
  return os;
}
// vector
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
  }
  os << "}";
  return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << *itr;
    itr++;
    if (itr != map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
// set
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) {
  os << "{";
  repi(itr, set_var) {
    os << *itr;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}

#define DUMPOUT cerr

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  DUMPOUT << head;
  if (sizeof...(Tail) > 0) {
    DUMPOUT << ", ";
  }
  dump_func(std::move(tail)...);
}
#ifdef DEBUG
#define DEB
#define dump(...)                                                              \
  DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                              \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "    ",                                                           \
      dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

#endif

int modpow(int a, int k, int p) {
  int ans = 1;
  while (k > 0) {
    if (k % 2 == 1) {
      ans *= a;
      ans %= p;
    }
    a = (a * a) % p;
    k = k / 2;
  }
  return ans;
}

// ax+by=1の解
int extgcd(int a, int b, int &x, int &y) {
  int d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}

// mod mでのaの逆数
int mod_inv(int a, int m) {
  int x, y;
  extgcd(a, m, x, y);
  return (m + x % m) % m;
}

// nの階乗 mod p
int factorial(int n, int p) {
  int r = 1;
  for (int i = 1; i <= n; i++)
    r = (r * i) % p;
  return r;
}

// nPr mod p
int permutation(int n, int r, int p) {
  int a = 1;
  for (int i = n - r + 1; i <= n; i++)
    a = (a * i) % p;
  return a;
}

// 階乗の事前計算
const int MAX = 623456;
int fac[MAX], fac_inv[MAX];

void make_table(int N) {
  fac[0] = fac[1] = 1;
  for (int i = 2; i <= N; i++)
    fac[i] = (fac[i - 1] * i) % mod;
  for (int i = 0; i <= N; i++)
    fac_inv[i] = mod_inv(fac[i], mod);
}

int combination(int n, int r, int p) {
  return (fac[n] * fac_inv[r]) % p * fac_inv[n - r] % p;
}

void solve() {
  int n, k;
  cin >> n >> k;

  make_table(523456);

  if (k >= n) {
    cout << combination(2 * n - 1, n, mod) << endl;
    return;
  }

  int ans = 0;
  // zeroの数を固定
  for (int x = 0; x <= k; x++) {
    ans += combination(n, x, mod) * combination(n - 1, n - x - 1, mod) % mod;
    ans %= mod;
  }
  cout << ans << endl;
}

signed main() {
  cout << fixed << setprecision(18);
  cerr << fixed << setprecision(18);
  solve();
}