Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
const int inf = 1e9 + 7;
const ll INF = 1e18;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
const int mod = 1000000007;
ll modpow(ll a, int b) {
  return b ? modpow(a * a % mod, b / 2) * (b % 2 ? a : 1) % mod : 1;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (auto &vi : vec)
    os << vi << " ";
  return os;
}

void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  int T;
  // cin >> T;
  T = 1;

  while (T--) {
    solve();
  }
}

const int MAX = 1000005;
ll fac[MAX], finv[MAX], inv[MAX];
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
// 二項係数計算
ll COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

ll f(int r, int c) {
  return ((r + 2) * COM(c + r + 2, r + 2) % mod - c - 1 + mod) % mod *
         modpow(c + 1, mod - 2) % mod;
}

// https://www.wolframalpha.com/input/?i=Sum%5BSum%5Bcomb%28x+%2B+y%2C+x%29%2C+%7By%2C+0%2C+c%7D%5D%2C+%7Bx%2C+0%2C+r%7D%5D&lang=ja
void solve() {
  int r1, r2, c1, c2;
  cin >> r1 >> c1 >> r2 >> c2;

  COMinit();

  ll ans =
      f(r2, c2) + mod - f(r2, c1 - 1) + mod - f(r1 - 1, c2) + f(r1 - 1, c1 - 1);
  cout << ans % mod << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
const int inf = 1e9 + 7;
const ll INF = 1e18;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
const int mod = 1000000007;
ll modpow(ll a, int b) {
  return b ? modpow(a * a % mod, b / 2) * (b % 2 ? a : 1) % mod : 1;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (auto &vi : vec)
    os << vi << " ";
  return os;
}

void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  int T;
  // cin >> T;
  T = 1;

  while (T--) {
    solve();
  }
}

const int MAX = 2000005; // c + r
ll fac[MAX], finv[MAX], inv[MAX];
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
// 二項係数計算
ll COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

ll f(int r, int c) {
  return ((r + 2) * COM(c + r + 2, r + 2) % mod - c - 1 + mod) % mod *
         modpow(c + 1, mod - 2) % mod;
}

// https://www.wolframalpha.com/input/?i=Sum%5BSum%5Bcomb%28x+%2B+y%2C+x%29%2C+%7By%2C+0%2C+c%7D%5D%2C+%7Bx%2C+0%2C+r%7D%5D&lang=ja
void solve() {
  int r1, r2, c1, c2;
  cin >> r1 >> c1 >> r2 >> c2;

  COMinit();

  ll ans =
      f(r2, c2) + mod - f(r2, c1 - 1) + mod - f(r1 - 1, c2) + f(r1 - 1, c1 - 1);
  cout << ans % mod << endl;
}