Problem p02862 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;

#define All(v) (v).begin(), (v).end()
int dy[4] = {-1, 0, 1, 0};
int dx[4] = {0, 1, 0, -1};
int Dy[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
int Dx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int mod = 1000000007;
const int inf = mod * mod;
const int d5 = 1000000;

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}
  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }

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

  ModInt pow(int e) {
    long long a = 1, p = x;
    while (e > 0) {
      if (e % 2 == 0) {
        p = (p * p) % mod;
        e /= 2;
      } else {
        a = (a * p) % mod;
        e--;
      }
    }
    return ModInt(a);
  }

  friend ostream &operator<<(ostream &os, const ModInt<mod> &p) {
    return os << p.x;
  }
  friend istream &operator>>(istream &is, ModInt<mod> &a) {
    long long x;
    is >> x;
    a = ModInt<mod>(x);
    return (is);
  }
};

using modint = ModInt<mod>;
modint f[d5];

void makef() {
  f[0] = 1;
  for (int i = 1; i < d5; i++)
    f[i] = f[i - 1] * i;
}

modint nCr(int n, int r) {
  if (n < r)
    return 0;
  return f[n] / (f[r] * f[n - r]);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int x, y;
  cin >> x >> y;
  if ((x + y) % 3 != 0) {
    cout << 0 << endl;
    return 0;
  }
  int a = (2 * x - y) / 3;
  int b = (2 * y - x) / 3;
  makef();
  cout << nCr(a + b, b) << endl;
}


Fixed submission:
#include <algorithm>
#include <cmath>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;

#define All(v) (v).begin(), (v).end()
int dy[4] = {-1, 0, 1, 0};
int dx[4] = {0, 1, 0, -1};
int Dy[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
int Dx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int mod = 1000000007;
const int inf = mod * mod;
const int d5 = 1000000;

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}
  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }

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

  ModInt pow(int e) {
    long long a = 1, p = x;
    while (e > 0) {
      if (e % 2 == 0) {
        p = (p * p) % mod;
        e /= 2;
      } else {
        a = (a * p) % mod;
        e--;
      }
    }
    return ModInt(a);
  }

  friend ostream &operator<<(ostream &os, const ModInt<mod> &p) {
    return os << p.x;
  }
  friend istream &operator>>(istream &is, ModInt<mod> &a) {
    long long x;
    is >> x;
    a = ModInt<mod>(x);
    return (is);
  }
};

using modint = ModInt<mod>;
modint f[d5];

void makef() {
  f[0] = 1;
  for (int i = 1; i < d5; i++)
    f[i] = f[i - 1] * i;
}

modint nCr(int n, int r) {
  if (n < r)
    return 0;
  else if (r < 0)
    return 0;
  return f[n] / (f[r] * f[n - r]);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int x, y;
  cin >> x >> y;
  if ((x + y) % 3 != 0) {
    cout << 0 << endl;
    return 0;
  }
  int a = (2 * x - y) / 3;
  int b = (2 * y - x) / 3;
  makef();
  cout << nCr(a + b, b) << endl;
}