Problem p02862 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) for (int i = (0); i < (n); ++i)
#define rng(x) (x).begin(), (x).end()
#define rrng(x) (x).rbegin(), (x).rend()
#define debug(x) cout << #x << " = " << (x) << endl;
#define SUM(x) accumulate(rng(x), 0)
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define uni(x) x.erase(unique(rng(x)), x.end())
#define SP << " " <<
using namespace std;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using pii = pair<int, int>;
using tiii = tuple<int, int, int>;
const int mod = 1000000007;
const double EPS = 1e-9;
const int INF = 1 << 30;
const ll INFLL = 1LL << 60;
// cout << fixed << setprecision(10);

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

// const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll x = 0) : x(x % mod) {}
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
} comb(500005);
// comb(n,k)ならcomb(n+k+5)とかにしておく

mint f(int n, int k) {
  if (n < 0)
    return 0;
  mint res = comb(n, k);
  // nPk = nCk * k!
  // res *= comb.fact[k]; // <- nPk求めたいときは必要
  return res;
}
// f(n,k).x で nCk か nPk が計算される

int main() {
  int x, y;
  cin >> x >> y;

  if ((x + y) % 3 != 0) {
    cout << 0 << endl;
    return 0;
  }

  if (min(x, y) < (x + y) / 3) {
    cout << 0 << endl;
    return 0;
  }

  int n = (x + y) / 3;
  int k = min(x, y) - n;
  mint ans = f(n, k);
  cout << ans.x << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) for (int i = (0); i < (n); ++i)
#define rng(x) (x).begin(), (x).end()
#define rrng(x) (x).rbegin(), (x).rend()
#define debug(x) cout << #x << " = " << (x) << endl;
#define SUM(x) accumulate(rng(x), 0)
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define uni(x) x.erase(unique(rng(x)), x.end())
#define SP << " " <<
using namespace std;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using pii = pair<int, int>;
using tiii = tuple<int, int, int>;
const int mod = 1000000007;
const double EPS = 1e-9;
const int INF = 1 << 30;
const ll INFLL = 1LL << 60;
// cout << fixed << setprecision(10);

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

// const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll x = 0) : x(x % mod) {}
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
} comb(2000005);
// comb(n,k)ならcomb(n+k+5)とかにしておく

mint f(int n, int k) {
  if (n < 0)
    return 0;
  mint res = comb(n, k);
  // nPk = nCk * k!
  // res *= comb.fact[k]; // <- nPk求めたいときは必要
  return res;
}
// f(n,k).x で nCk か nPk が計算される

int main() {
  int x, y;
  cin >> x >> y;

  if ((x + y) % 3 != 0) {
    cout << 0 << endl;
    return 0;
  }

  if (min(x, y) < (x + y) / 3) {
    cout << 0 << endl;
    return 0;
  }

  int n = (x + y) / 3;
  int k = min(x, y) - n;
  mint ans = f(n, k);
  cout << ans.x << endl;
  return 0;
}