Problem p02862 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define rrng(a) a.rbegin(), a.rend()
#define maxs(x, y) (x = max(x, y))
#define mins(x, y) (x = min(x, y))
#define limit(x, l, r) max(l, min(x, r))
#define lims(x, l, r) (x = max(l, min(x, r)))
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)), x.end())
#define show(x) cout << #x << " = " << x << endl;
#define print(x) cout << x << endl;
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef tuple<int, int, int> T;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
typedef vector<T> vt;
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  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;
  }
};
mint comb(int n, int r) {
  if (n - r < r)
    r = n - r;
  if (r == 0)
    return 1;
  if (r == 1)
    return n;

  vector<mint> numerator(r, 0), denominator(r, 0);
  for (int k = 0; k < r; k++) {
    numerator[k] = n - r + k + 1;
    denominator[k] = k + 1;
  }

  for (int p = 2; p < r + 1; p++) {
    mint pivot = denominator[p - 1];
    if (pivot.x > 1) {
      int offset = (n - r) % p;
      for (int k = p - 1; k < r; k += p) {
        numerator[k - offset] /= pivot;
        denominator[k] /= pivot;
      }
    }
  }

  mint res = 1;
  for (int k = 0; k < r; k++) {
    if (numerator[k].x > 1) {
      res *= numerator[k];
    }
  }
  return res;
}

int main() {
  ll x, y;
  cin >> x >> y;
  ll b = (2 * x - y) / 3;
  ll a = x - 2 * b;
  mint ans = comb(a + b, a);
  if ((a + 2 * b != x) | (2 * a + b != y))
    ans = 0;
  cout << ans.x << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define rrng(a) a.rbegin(), a.rend()
#define maxs(x, y) (x = max(x, y))
#define mins(x, y) (x = min(x, y))
#define limit(x, l, r) max(l, min(x, r))
#define lims(x, l, r) (x = max(l, min(x, r)))
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)), x.end())
#define show(x) cout << #x << " = " << x << endl;
#define print(x) cout << x << endl;
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef tuple<int, int, int> T;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
typedef vector<T> vt;
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  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;
  }
};
mint comb(int n, int r) {
  if (n - r < r)
    r = n - r;
  if (r == 0)
    return 1;
  if (r == 1)
    return n;

  vector<mint> numerator(r, 0), denominator(r, 0);
  for (int k = 0; k < r; k++) {
    numerator[k] = n - r + k + 1;
    denominator[k] = k + 1;
  }

  for (int p = 2; p < r + 1; p++) {
    mint pivot = denominator[p - 1];
    if (pivot.x > 1) {
      int offset = (n - r) % p;
      for (int k = p - 1; k < r; k += p) {
        numerator[k - offset] /= pivot;
        denominator[k] /= pivot;
      }
    }
  }

  mint res = 1;
  for (int k = 0; k < r; k++) {
    if (numerator[k].x > 1) {
      res *= numerator[k];
    }
  }
  return res;
}

int main() {
  ll x, y;
  cin >> x >> y;
  ll b = (2 * x - y) / 3;
  ll a = x - 2 * b;
  mint ans = 0;
  if (a >= 0 and b >= 0)
    ans = comb(a + b, a);
  if ((a + 2 * b != x) | (2 * a + b != y))
    ans = 0;
  cout << ans.x << endl;
  return 0;
}