Problem p02769 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
class ModInt {
  static int MOD;
  long long x;

public:
  static void mod(int m) { MOD = m; }

  ModInt(long long v = 0) : x(v % MOD) {
    if (x < 0)
      x += MOD;
  }

  ModInt &operator+=(const ModInt &r) {
    x += r.x;
    if (x >= MOD)
      x -= MOD;
    return *this;
  }

  ModInt &operator-=(const ModInt &r) {
    x -= r.x;
    if (x < 0)
      x += MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt &r) {
    x = (x * r.x) % MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt &r) {
    *this *= r.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

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

  ModInt inverse() const {
    long long a = x, b = MOD, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(long long n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &x) { return os << x.x; }

  friend istream &operator>>(istream &is, ModInt &x) { return is >> x.x; }

  int getMod() { return MOD; }

  long long val() { return x; }
};
int ModInt::MOD = 1000000007;
template <typename T> class Combination {
  vector<T> fact, ifact;

public:
  Combination(int nMax) : fact(nMax + 1), ifact(nMax + 1) {
    fact[0] = 1;
    for (int i = 0; i < nMax; ++i)
      fact[i + 1] = fact[i] * (i + 1);
    for (int i = 0; i < nMax + 1; ++i)
      ifact[i] = fact[i].inverse();
  }

  T factorial(int n) const { return fact[n]; }
  T inverseFactorial(int n) const { return ifact[n]; }

  T P(int n, int k) const {
    if (n < 0 || k < 0 || n < k)
      return 0;
    return factorial(n) * inverseFactorial(n - k);
  }

  T C(int n, int k) const {
    if (n < 0 || k < 0 || n < k)
      return 0;
    return factorial(n) * inverseFactorial(n - k) * inverseFactorial(k);
  }

  T H(int n, int k) const {
    if (n < 0 || k < 0)
      return 0;
    return k == 0 ? 1 : C(n + k - 1, k);
  }
};

int main() {
  int n, k;
  cin >> n >> k;
  Combination<ModInt> comb(n);

  ModInt ans = 0;
  for (int i = 0; i <= k; ++i) {
    ans += comb.C(n, i) * comb.C(n - 1, i);
  }
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
class ModInt {
  static int MOD;
  long long x;

public:
  static void mod(int m) { MOD = m; }

  ModInt(long long v = 0) : x(v % MOD) {
    if (x < 0)
      x += MOD;
  }

  ModInt &operator+=(const ModInt &r) {
    x += r.x;
    if (x >= MOD)
      x -= MOD;
    return *this;
  }

  ModInt &operator-=(const ModInt &r) {
    x -= r.x;
    if (x < 0)
      x += MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt &r) {
    x = (x * r.x) % MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt &r) {
    *this *= r.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

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

  ModInt inverse() const {
    long long a = x, b = MOD, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(long long n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &x) { return os << x.x; }

  friend istream &operator>>(istream &is, ModInt &x) { return is >> x.x; }

  int getMod() { return MOD; }

  long long val() { return x; }
};
int ModInt::MOD = 1000000007;
template <typename T> class Combination {
  vector<T> fact, ifact;

public:
  Combination(int nMax) : fact(nMax + 1), ifact(nMax + 1) {
    fact[0] = 1;
    for (int i = 0; i < nMax; ++i)
      fact[i + 1] = fact[i] * (i + 1);
    for (int i = 0; i < nMax + 1; ++i)
      ifact[i] = fact[i].inverse();
  }

  T factorial(int n) const { return fact[n]; }
  T inverseFactorial(int n) const { return ifact[n]; }

  T P(int n, int k) const {
    if (n < 0 || k < 0 || n < k)
      return 0;
    return factorial(n) * inverseFactorial(n - k);
  }

  T C(int n, int k) const {
    if (n < 0 || k < 0 || n < k)
      return 0;
    return factorial(n) * inverseFactorial(n - k) * inverseFactorial(k);
  }

  T H(int n, int k) const {
    if (n < 0 || k < 0)
      return 0;
    return k == 0 ? 1 : C(n + k - 1, k);
  }
};

int main() {
  int n, k;
  cin >> n >> k;
  Combination<ModInt> comb(n);

  ModInt ans = 0;
  for (int i = 0; i <= min(k, n - 1); ++i) {
    ans += comb.C(n, i) * comb.C(n - 1, i);
  }
  cout << ans << endl;

  return 0;
}