Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using lint = long long;
using P = pair<int, int>;
using LLP = pair<long long, long long>;

#define REP(i, x, n) for (int i = (x), i##_len = (int)(n); i < i##_len; ++i)
#define rep(i, n) for (int i = 0, i##_len = (int)(n); i < i##_len; ++i)
#define reps(i, n) for (int i = 1, i##_len = (int)(n); i <= i##_len; ++i)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; --i)
#define rreps(i, n) for (int i = (int)(n); i > 0; --i)
#define SORT(x) sort((x).begin(), (x).end())
#define SORT_INV(x) sort((x).rbegin(), (x).rend())
#define REVERSE(x) reverse((x).begin(), (x).end())
#define TWINS(x) cout << ((x) ? "Yay!" : ":(") << '\n'

constexpr int IINF = (1 << 30) - 1;
constexpr long long LLINF = 1LL << 61;
constexpr double EPS = 1e-10;

constexpr int dx4[] = {1, 0, -1, 0}, dy4[] = {0, 1, 0, -1};
constexpr int dx8[] = {1, 1, 0, -1, -1, -1, 0, 1},
              dy8[] = {0, -1, -1, -1, 0, 1, 1, 1};

template <long long modulus> class ModInt {
  long long num;

  long long mod(long long n) {
    if (n < 0LL) {
      n = (n % modulus) + modulus;
    }
    return n % modulus;
  }

  long long ext_gcd(long long a, long long b, long long &x, long long &y) {
    if (b == 0LL) {
      x = 1;
      y = 0;
      return a;
    }

    long long d = ext_gcd(b, a % b, y, x);
    y -= a / b * x;

    return d;
  }

  long long get_inv(long long n) {
    long long x, y;
    ext_gcd(n, modulus, x, y);
    return mod(x);
  }

public:
  constexpr ModInt(const long long n = 0LL) noexcept : num(mod(n)) {}

  long long get(void) { return num; }

  ModInt &operator+=(const ModInt &rhs) {
    num += rhs.num;
    if (num >= modulus) {
      num -= modulus;
    }
    return *this;
  }

  ModInt &operator-=(const ModInt &rhs) {
    if (num < rhs.num) {
      num += modulus;
    }
    num -= rhs.num;
    return *this;
  }

  ModInt &operator*=(const ModInt &rhs) {
    num = num * rhs.num % modulus;
    return *this;
  }

  ModInt &operator/=(const ModInt &rhs) {
    long long inv = get_inv(rhs.num);
    num = num * inv % modulus;
    return *this;
  }

  template <typename T> ModInt &operator+=(const T &rhs) {
    return *this += ModInt(rhs);
  }

  template <typename T> ModInt &operator-=(const T &rhs) {
    return *this -= ModInt(rhs);
  }

  template <typename T> ModInt &operator*=(const T &rhs) {
    return *this *= ModInt(rhs);
  }

  template <typename T> ModInt &operator/=(const T &rhs) {
    return *this /= ModInt(rhs);
  }

  template <typename T> ModInt operator+(const T &rhs) const {
    return ModInt(*this) += ModInt(rhs);
  }

  template <typename T> ModInt operator-(const T &rhs) const {
    return ModInt(*this) -= ModInt(rhs);
  }

  template <typename T> ModInt operator*(const T &rhs) const {
    return ModInt(*this) *= ModInt(rhs);
  }

  template <typename T> ModInt operator/(const T &rhs) const {
    return ModInt(*this) /= ModInt(rhs);
  }

  ModInt &operator=(const ModInt &rhs) {
    num = rhs.num;
    return *this;
  }

  template <typename T> ModInt &operator=(const T &rhs) {
    num = mod(rhs);
    return *this;
  }

  bool operator<(const ModInt &rhs) const { return num < rhs.num; }
};

template <long long modulus>
std::istream &operator>>(std::istream &lhs, ModInt<modulus> &rhs) {
  long long val;
  lhs >> val;
  rhs = val;
  return lhs;
}

template <long long modulus>
std::ostream &operator<<(std::ostream &lhs, ModInt<modulus> rhs) {
  lhs << rhs.get();
  return lhs;
}

using mint = ModInt<1000000007LL>;

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

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

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;

  constexpr int m = 1010101;

  vector<mint> fact(m + 1);
  fact[0] = 1;
  reps(i, m) { fact[i] = fact[i - 1] * i; }

  vector<mint> inv(m + 1);
  inv[m] = mint(1) / fact[m];
  rreps(i, m) { inv[i - 1] = inv[i] * i; }

  auto comb = [&](int r, int c) { return fact[r] * inv[c] * inv[r - c]; };

  mint ans = comb(r2 + c2 + 2, r2 + 1) - comb(r2 + c1 + 1, r2 + 1) -
             comb(r1 + c2 + 1, c2 + 1) + comb(r1 + c1, r1);
  cout << ans << endl;

  cout << flush;

  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using lint = long long;
using P = pair<int, int>;
using LLP = pair<long long, long long>;

#define REP(i, x, n) for (int i = (x), i##_len = (int)(n); i < i##_len; ++i)
#define rep(i, n) for (int i = 0, i##_len = (int)(n); i < i##_len; ++i)
#define reps(i, n) for (int i = 1, i##_len = (int)(n); i <= i##_len; ++i)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; --i)
#define rreps(i, n) for (int i = (int)(n); i > 0; --i)
#define SORT(x) sort((x).begin(), (x).end())
#define SORT_INV(x) sort((x).rbegin(), (x).rend())
#define REVERSE(x) reverse((x).begin(), (x).end())
#define TWINS(x) cout << ((x) ? "Yay!" : ":(") << '\n'

constexpr int IINF = (1 << 30) - 1;
constexpr long long LLINF = 1LL << 61;
constexpr double EPS = 1e-10;

constexpr int dx4[] = {1, 0, -1, 0}, dy4[] = {0, 1, 0, -1};
constexpr int dx8[] = {1, 1, 0, -1, -1, -1, 0, 1},
              dy8[] = {0, -1, -1, -1, 0, 1, 1, 1};

template <long long modulus> class ModInt {
  long long num;

  long long mod(long long n) {
    if (n < 0LL) {
      n = (n % modulus) + modulus;
    }
    return n % modulus;
  }

  long long ext_gcd(long long a, long long b, long long &x, long long &y) {
    if (b == 0LL) {
      x = 1;
      y = 0;
      return a;
    }

    long long d = ext_gcd(b, a % b, y, x);
    y -= a / b * x;

    return d;
  }

  long long get_inv(long long n) {
    long long x, y;
    ext_gcd(n, modulus, x, y);
    return mod(x);
  }

public:
  constexpr ModInt(const long long n = 0LL) noexcept : num(mod(n)) {}

  long long get(void) { return num; }

  ModInt &operator+=(const ModInt &rhs) {
    num += rhs.num;
    if (num >= modulus) {
      num -= modulus;
    }
    return *this;
  }

  ModInt &operator-=(const ModInt &rhs) {
    if (num < rhs.num) {
      num += modulus;
    }
    num -= rhs.num;
    return *this;
  }

  ModInt &operator*=(const ModInt &rhs) {
    num = num * rhs.num % modulus;
    return *this;
  }

  ModInt &operator/=(const ModInt &rhs) {
    long long inv = get_inv(rhs.num);
    num = num * inv % modulus;
    return *this;
  }

  template <typename T> ModInt &operator+=(const T &rhs) {
    return *this += ModInt(rhs);
  }

  template <typename T> ModInt &operator-=(const T &rhs) {
    return *this -= ModInt(rhs);
  }

  template <typename T> ModInt &operator*=(const T &rhs) {
    return *this *= ModInt(rhs);
  }

  template <typename T> ModInt &operator/=(const T &rhs) {
    return *this /= ModInt(rhs);
  }

  template <typename T> ModInt operator+(const T &rhs) const {
    return ModInt(*this) += ModInt(rhs);
  }

  template <typename T> ModInt operator-(const T &rhs) const {
    return ModInt(*this) -= ModInt(rhs);
  }

  template <typename T> ModInt operator*(const T &rhs) const {
    return ModInt(*this) *= ModInt(rhs);
  }

  template <typename T> ModInt operator/(const T &rhs) const {
    return ModInt(*this) /= ModInt(rhs);
  }

  ModInt &operator=(const ModInt &rhs) {
    num = rhs.num;
    return *this;
  }

  template <typename T> ModInt &operator=(const T &rhs) {
    num = mod(rhs);
    return *this;
  }

  bool operator<(const ModInt &rhs) const { return num < rhs.num; }
};

template <long long modulus>
std::istream &operator>>(std::istream &lhs, ModInt<modulus> &rhs) {
  long long val;
  lhs >> val;
  rhs = val;
  return lhs;
}

template <long long modulus>
std::ostream &operator<<(std::ostream &lhs, ModInt<modulus> rhs) {
  lhs << rhs.get();
  return lhs;
}

using mint = ModInt<1000000007LL>;

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

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

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;

  constexpr int m = 2020202;

  vector<mint> fact(m + 1);
  fact[0] = 1;
  reps(i, m) { fact[i] = fact[i - 1] * i; }

  vector<mint> inv(m + 1);
  inv[m] = mint(1) / fact[m];
  rreps(i, m) { inv[i - 1] = inv[i] * i; }

  auto comb = [&](int r, int c) { return fact[r] * inv[c] * inv[r - c]; };

  mint ans = comb(r2 + c2 + 2, r2 + 1) - comb(r2 + c1 + 1, r2 + 1) -
             comb(r1 + c2 + 1, c2 + 1) + comb(r1 + c1, r1);
  cout << ans << endl;

  cout << flush;

  return 0;
}