Problem p02862 (C++)

Buggy submission (status: Memory Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, j, n) for (int i = j; i < n; i++)
#define all(x) (x).begin(), (x).end()
#define pii pair<int, int>
#define pll pair<ll, ll>
#define vc vector
#define a first
#define b second
#define pb pusu_back
#define INF (1000000000)
#define MOD (1000000007)
#define MAX (100000000)

/////////////////////////////////////////////////////////

class XY {
public:
  ll x, y;
  XY() { x = y = 0; }
  XY(ll u, ll v) {
    x = u;
    y = v;
  }
};

template <typename T1, typename T2> void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}
template <typename T1, typename T2> void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}

template <typename T1, typename T2> ll mypow(T1 a, T2 n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return a;
  if (n % 2)
    return a * mypow(a, n - 1);
  ll tmp = mypow(a, n / 2);
  return tmp * tmp;
}

template <typename T> int BS(vector<T> &V, int left, int right, T key) {
  int mid = (left + right) / 2;
  if (V[mid] <= key)
    left = mid;
  else
    right = mid;
  if (right - mid == 1)
    return left;
  else
    return BS(V, left, right, key);
}

ll comb(ll n, ll r) {
  ll res = 1;
  rep(i, 0, r) {
    res *= n - i;
    res /= i + 1;
  }
  return res;
}

template <typename T> T euclid(T a, T b) {
  if (b == 0)
    return a;
  else
    return euclid(b, a % b);
}

/////////////////////////////////////////////////////////

// MOD of Combination

vector<ll> fact(MAX, 0), fact_inv(MAX, 0);

ll pow_mod(ll a, ll b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return res;
}

void prepare_comb_mod(ll n) {
  fact[0] = 1LL;
  for (ll i = 0; i < n; i++)
    fact[i + 1] = fact[i] * (i + 1) % MOD;
  fact_inv[n] = pow_mod(fact[n], MOD - 2);
  for (ll i = n - 1; i >= 0; i--)
    fact_inv[i] = fact_inv[i + 1] * (i + 1) % MOD;
}

ll comb_mod(ll n, ll r) {
  return (fact[n] * fact_inv[r]) % MOD * fact_inv[n - r] % MOD;
}

/////////////////////////////////////////////////////////

// Union-Find Tree

template <class T> struct UFT {
  vector<int> par;
  vector<int> rank;
  vector<T> diff_weight;

  UFT(int n = 1, T w = 0) { init(n, w); }

  void init(int n = 1, T w = 0) {
    par.resize(n);
    rank.resize(n);
    diff_weight.resize(n);
    rep(i, 0, n) {
      par[i] = i;
      rank[i] = 0;
      diff_weight[i] = w;
    }
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      diff_weight[x] += diff_weight[par[x]];
      return par[x] = r;
    }
  }

  bool unite(int x, int y, int w = 0) {
    w += weight(x);
    w -= weight(y);
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y]) {
      par[x] = y;
      diff_weight[x] = -w;
    } else {
      par[y] = x;
      diff_weight[y] = w;
      if (rank[x] == rank[y])
        rank[x]++;
    }
    return true;
  }

  bool same(int x, int y) { return root(x) == root(y); }

  T weight(int x) {
    root(x);
    return diff_weight[x];
  }

  T diff(int x, int y) { return weight(y) - weight(x); }
};

/////////////////////////////////////////////////////////

void Main() {
  ll x, y;
  cin >> x >> y;
  prepare_comb_mod(x + y);
  ll res = 0;
  rep(n, 0, x + 1) {
    if ((x - n) % 2 == 0 && (x - n) / 2 == y - 2 * n && (x - n) / 2 >= 0) {
      int m = (x - n) / 2;
      res += comb_mod(n + m, n);
      res %= MOD;
    }
  }
  cout << res << endl;
}

/////////////////////////////////////////////////////////

int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << std::fixed << std::setprecision(15);

  Main();
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, j, n) for (int i = j; i < n; i++)
#define all(x) (x).begin(), (x).end()
#define pii pair<int, int>
#define pll pair<ll, ll>
#define vc vector
#define a first
#define b second
#define pb pusu_back
#define INF (1000000000)
#define MOD (1000000007)
#define MAX (5000000)

/////////////////////////////////////////////////////////

class XY {
public:
  ll x, y;
  XY() { x = y = 0; }
  XY(ll u, ll v) {
    x = u;
    y = v;
  }
};

template <typename T1, typename T2> void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}
template <typename T1, typename T2> void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}

template <typename T1, typename T2> ll mypow(T1 a, T2 n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return a;
  if (n % 2)
    return a * mypow(a, n - 1);
  ll tmp = mypow(a, n / 2);
  return tmp * tmp;
}

template <typename T> int BS(vector<T> &V, int left, int right, T key) {
  int mid = (left + right) / 2;
  if (V[mid] <= key)
    left = mid;
  else
    right = mid;
  if (right - mid == 1)
    return left;
  else
    return BS(V, left, right, key);
}

ll comb(ll n, ll r) {
  ll res = 1;
  rep(i, 0, r) {
    res *= n - i;
    res /= i + 1;
  }
  return res;
}

template <typename T> T euclid(T a, T b) {
  if (b == 0)
    return a;
  else
    return euclid(b, a % b);
}

/////////////////////////////////////////////////////////

// MOD of Combination

vector<ll> fact(MAX, 0), fact_inv(MAX, 0);

ll pow_mod(ll a, ll b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return res;
}

void prepare_comb_mod(ll n) {
  fact[0] = 1LL;
  for (ll i = 0; i < n; i++)
    fact[i + 1] = fact[i] * (i + 1) % MOD;
  fact_inv[n] = pow_mod(fact[n], MOD - 2);
  for (ll i = n - 1; i >= 0; i--)
    fact_inv[i] = fact_inv[i + 1] * (i + 1) % MOD;
}

ll comb_mod(ll n, ll r) {
  return (fact[n] * fact_inv[r]) % MOD * fact_inv[n - r] % MOD;
}

/////////////////////////////////////////////////////////

// Union-Find Tree

template <class T> struct UFT {
  vector<int> par;
  vector<int> rank;
  vector<T> diff_weight;

  UFT(int n = 1, T w = 0) { init(n, w); }

  void init(int n = 1, T w = 0) {
    par.resize(n);
    rank.resize(n);
    diff_weight.resize(n);
    rep(i, 0, n) {
      par[i] = i;
      rank[i] = 0;
      diff_weight[i] = w;
    }
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      diff_weight[x] += diff_weight[par[x]];
      return par[x] = r;
    }
  }

  bool unite(int x, int y, int w = 0) {
    w += weight(x);
    w -= weight(y);
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y]) {
      par[x] = y;
      diff_weight[x] = -w;
    } else {
      par[y] = x;
      diff_weight[y] = w;
      if (rank[x] == rank[y])
        rank[x]++;
    }
    return true;
  }

  bool same(int x, int y) { return root(x) == root(y); }

  T weight(int x) {
    root(x);
    return diff_weight[x];
  }

  T diff(int x, int y) { return weight(y) - weight(x); }
};

/////////////////////////////////////////////////////////

void Main() {
  ll x, y;
  cin >> x >> y;
  prepare_comb_mod(x + y);
  ll res = 0;
  rep(n, 0, x + 1) {
    if ((x - n) % 2 == 0 && (x - n) / 2 == y - 2 * n && (x - n) / 2 >= 0) {
      int m = (x - n) / 2;
      res += comb_mod(n + m, n);
      res %= MOD;
    }
  }
  cout << res << endl;
}

/////////////////////////////////////////////////////////

int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << std::fixed << std::setprecision(15);

  Main();
}