Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <assert.h>
#include <bitset>
#include <climits>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#define LEN(x) (long long)(x.size())
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define FOE(i, a) for (auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v, x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT_COUNT(bit) (__builtin_popcount(bit))

typedef long long LL;
template <typename T> std::vector<T> make_v(size_t a) {
  return std::vector<T>(a);
}
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return std::vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
} // C++14
template <typename T, typename V>
typename std::enable_if<std::is_class<T>::value == 0>::type fill_v(T &t,
                                                                   const V &v) {
  t = v;
}
template <typename T, typename V>
typename std::enable_if<std::is_class<T>::value != 0>::type fill_v(T &t,
                                                                   const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}
template <class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
void print() { std::cout << std::endl; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
  std::cout << head;
  if (sizeof...(tail) != 0) {
    std::cout << " ";
  }
  print(std::forward<Tail>(tail)...);
}
template <class T> void print(std::vector<T> &v) {
  for (auto &a : v) {
    std::cout << a;
    if (&a != &v.back()) {
      std::cout << " ";
    }
  }
  std::cout << std::endl;
}
template <class T> void print(std::vector<std::vector<T>> &vv) {
  for (auto &v : vv) {
    print(v);
  }
}
void debug() { std::cerr << std::endl; }
template <class Head, class... Tail> void debug(Head &&head, Tail &&...tail) {
  std::cerr << head;
  if (sizeof...(tail) != 0) {
    std::cerr << " ";
  }
  print(std::forward<Tail>(tail)...);
}
template <class T> void debug(std::vector<T> &v) {
  for (auto &a : v) {
    std::cerr << a;
    if (&a != &v.back()) {
      std::cerr << " ";
    }
  }
  std::cerr << std::endl;
}
template <class T> void debug(std::vector<std::vector<T>> &vv) {
  for (auto &v : vv) {
    print(v);
  }
}
inline bool inside(long long y, long long x, long long H, long long W) {
  return 0 <= y and y < H and 0 <= x and x < W;
}
template <class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) {
  return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}
template <class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) {
  return abs(x1 - x2) + abs(y1 - y2);
}
template <typename T> T &chmin(T &a, const T &b) { return a = std::min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = std::max(a, b); }
bool is_bit_on(const unsigned long long bit, const unsigned int i) {
  return (bit >> i) & 1u;
}
unsigned long long bit_set(const unsigned long long bit, const unsigned int i,
                           const unsigned int b) {
  assert(b == 0 or b == 1);
  if (b == 0) {
    return bit & ~(1ull << i);
  } else {
    return bit | (1ull << i);
  }
}

// 初項s交差d長さnの数列の和
long long sum_of_arithmetic_progression(long long s, long long d, long long n) {
  return n * (2 * s + (n - 1) * d) / 2;
}

// xが2の階乗かどうか判定
bool is_power_of_two(long long x) { return !(x & (x - 1)); }

long long gcd(long long a, long long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}

long long gcd(std::vector<long long> &v) {
  long long ans = v[0];
  for (int i = 1; i < (int)v.size(); ++i) {
    ans = gcd(ans, v[i]);
  }
  return ans;
}

long long lcm(long long a, long long b) {
  long long g = gcd(a, b);
  return a / g * b;
}

const int INF = 1u << 30u; // 1,073,741,824
const long long LINF = 1ull << 60u;
const double EPS = 1e-9;
const long double PI = acos(-1.0);
const std::vector<int> dy2 = {0, 1}, dx2 = {1, 0};
const std::vector<int> dy4 = {0, 1, 0, -1}, dx4 = {1, 0, -1, 0};
const std::vector<int> dy8 = {0, -1, 0, 1, 1, -1, -1, 1},
                       dx8 = {1, 0, -1, 0, 1, 1, -1, -1};

using namespace std;

template <int mod> struct mint {
  long long x;
  mint(long long 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(long long 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;
  }
};

long long modpow(long long base, long long exp, long long mod) {
  base %= mod;
  long long result = 1;
  while (exp > 0) {
    if (exp & 1) {
      result = (result * base) % mod;
    }
    base = (base * base) % mod;
    exp >>= 1;
  }
  return result;
}

long long combination(long long n, long long r) {
  uint64_t ans = 1;
  for (uint64_t d = 1; d <= r; ++d) {
    ans *= n--;
    ans /= d;
  }
  return ans;
}

class Combination {
public:
  int mod;
  vector<long long> fact;
  vector<long long> inv;

  Combination(int n, int mod) : mod(mod) {
    assert(0 < n);
    this->fact.resize(n + 1, 1);
    this->inv.resize(n + 1, 1);

    for (int i = 1; i < fact.size(); ++i) {
      this->fact[i] = (i * this->fact[i - 1]) % this->mod;
      this->inv[i] = modpow(this->fact[i], this->mod - 2, this->mod);
    }
  }

  long long nCr(int n, int k) {
    assert(0 <= n and n < this->fact.size() and 0 <= k);
    assert(n >= k);
    auto a = this->fact[n];
    auto b = (this->inv[k] * this->inv[n - k]) % this->mod;
    return (a * b) % this->mod;
  }

  long long nHr(int n, int k) { return nCr(n + k - 1, k); }
};

const int MOD = 1000000000 + 7;

Combination comb(1000100, MOD);

mint<MOD> solve(LL Y1, LL X1, LL Y2, LL X2) {
  const auto H = Y2 - Y1 + 1;
  const auto W = X2 - X1 + 1;

  mint<MOD> ans = comb.nCr(Y1 + X1, Y1);

  FOR(y, 1, H) {
    auto a = comb.nCr(Y1 + y + X1, X1);
    ans += a;

    LL m = (H - y);
    LL n = (W - 1);
    if (n == 0 or m == 0) {
      continue;
    }
    //        FOR(i, 0, m) {
    //            FOR(j, 0, n) {
    //                count += comb.nCr(i + j, i);
    //            }
    //        }
    mint<MOD> count = mint<MOD>(m + 1) * comb.nCr(m + n, n - 1) / n - 1;
    ans += count * a;
  }

  FOR(x, 1, W) {
    auto a = comb.nCr(X1 + x + Y1, Y1);
    ans += a;

    LL m = (H - 1);
    LL n = (W - x);
    if (n == 0 or m == 0) {
      continue;
    }
    //        FOR(i, 0, H - 1) {
    //            FOR(j, 0, W - x) {
    //                int c = comb.nCr(i + j, i);
    //                count += comb.nCr(i + j, i);
    //            }
    //        }
    mint<MOD> count = mint<MOD>(m + 1) * comb.nCr(m + n, n - 1) / n - 1;
    ans += count * a;
  }

  return ans;
}

mint<MOD> check(LL Y1, LL X1, LL Y2, LL X2) {
  mint<MOD> ans = 0;

  FOR(y, Y1, Y2 + 1) {
    FOR(x, X1, X2 + 1) { ans += comb.nCr(y + x, x); }
  }

  return ans;
}

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

  LL Y1, X1, Y2, X2;
  cin >> Y1 >> X1 >> Y2 >> X2;
  print(solve(Y1, X1, Y2, X2).x);

  return 0;
}


Fixed submission:
#include <algorithm>
#include <array>
#include <assert.h>
#include <bitset>
#include <climits>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#define LEN(x) (long long)(x.size())
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define FOE(i, a) for (auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v, x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT_COUNT(bit) (__builtin_popcount(bit))

typedef long long LL;
template <typename T> std::vector<T> make_v(size_t a) {
  return std::vector<T>(a);
}
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return std::vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
} // C++14
template <typename T, typename V>
typename std::enable_if<std::is_class<T>::value == 0>::type fill_v(T &t,
                                                                   const V &v) {
  t = v;
}
template <typename T, typename V>
typename std::enable_if<std::is_class<T>::value != 0>::type fill_v(T &t,
                                                                   const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}
template <class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
void print() { std::cout << std::endl; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
  std::cout << head;
  if (sizeof...(tail) != 0) {
    std::cout << " ";
  }
  print(std::forward<Tail>(tail)...);
}
template <class T> void print(std::vector<T> &v) {
  for (auto &a : v) {
    std::cout << a;
    if (&a != &v.back()) {
      std::cout << " ";
    }
  }
  std::cout << std::endl;
}
template <class T> void print(std::vector<std::vector<T>> &vv) {
  for (auto &v : vv) {
    print(v);
  }
}
void debug() { std::cerr << std::endl; }
template <class Head, class... Tail> void debug(Head &&head, Tail &&...tail) {
  std::cerr << head;
  if (sizeof...(tail) != 0) {
    std::cerr << " ";
  }
  print(std::forward<Tail>(tail)...);
}
template <class T> void debug(std::vector<T> &v) {
  for (auto &a : v) {
    std::cerr << a;
    if (&a != &v.back()) {
      std::cerr << " ";
    }
  }
  std::cerr << std::endl;
}
template <class T> void debug(std::vector<std::vector<T>> &vv) {
  for (auto &v : vv) {
    print(v);
  }
}
inline bool inside(long long y, long long x, long long H, long long W) {
  return 0 <= y and y < H and 0 <= x and x < W;
}
template <class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) {
  return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}
template <class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) {
  return abs(x1 - x2) + abs(y1 - y2);
}
template <typename T> T &chmin(T &a, const T &b) { return a = std::min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = std::max(a, b); }
bool is_bit_on(const unsigned long long bit, const unsigned int i) {
  return (bit >> i) & 1u;
}
unsigned long long bit_set(const unsigned long long bit, const unsigned int i,
                           const unsigned int b) {
  assert(b == 0 or b == 1);
  if (b == 0) {
    return bit & ~(1ull << i);
  } else {
    return bit | (1ull << i);
  }
}

// 初項s交差d長さnの数列の和
long long sum_of_arithmetic_progression(long long s, long long d, long long n) {
  return n * (2 * s + (n - 1) * d) / 2;
}

// xが2の階乗かどうか判定
bool is_power_of_two(long long x) { return !(x & (x - 1)); }

long long gcd(long long a, long long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}

long long gcd(std::vector<long long> &v) {
  long long ans = v[0];
  for (int i = 1; i < (int)v.size(); ++i) {
    ans = gcd(ans, v[i]);
  }
  return ans;
}

long long lcm(long long a, long long b) {
  long long g = gcd(a, b);
  return a / g * b;
}

const int INF = 1u << 30u; // 1,073,741,824
const long long LINF = 1ull << 60u;
const double EPS = 1e-9;
const long double PI = acos(-1.0);
const std::vector<int> dy2 = {0, 1}, dx2 = {1, 0};
const std::vector<int> dy4 = {0, 1, 0, -1}, dx4 = {1, 0, -1, 0};
const std::vector<int> dy8 = {0, -1, 0, 1, 1, -1, -1, 1},
                       dx8 = {1, 0, -1, 0, 1, 1, -1, -1};

using namespace std;

template <int mod> struct mint {
  long long x;
  mint(long long 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(long long 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;
  }
};

long long modpow(long long base, long long exp, long long mod) {
  base %= mod;
  long long result = 1;
  while (exp > 0) {
    if (exp & 1) {
      result = (result * base) % mod;
    }
    base = (base * base) % mod;
    exp >>= 1;
  }
  return result;
}

long long combination(long long n, long long r) {
  uint64_t ans = 1;
  for (uint64_t d = 1; d <= r; ++d) {
    ans *= n--;
    ans /= d;
  }
  return ans;
}

class Combination {
public:
  int mod;
  vector<long long> fact;
  vector<long long> inv;

  Combination(int n, int mod) : mod(mod) {
    assert(0 < n);
    this->fact.resize(n + 1, 1);
    this->inv.resize(n + 1, 1);

    for (int i = 1; i < fact.size(); ++i) {
      this->fact[i] = (i * this->fact[i - 1]) % this->mod;
      this->inv[i] = modpow(this->fact[i], this->mod - 2, this->mod);
    }
  }

  long long nCr(int n, int k) {
    assert(0 <= n and n < this->fact.size() and 0 <= k);
    assert(n >= k);
    auto a = this->fact[n];
    auto b = (this->inv[k] * this->inv[n - k]) % this->mod;
    return (a * b) % this->mod;
  }

  long long nHr(int n, int k) { return nCr(n + k - 1, k); }
};

const int MOD = 1000000000 + 7;

Combination comb(3000100, MOD);

mint<MOD> solve(LL Y1, LL X1, LL Y2, LL X2) {
  const auto H = Y2 - Y1 + 1;
  const auto W = X2 - X1 + 1;

  mint<MOD> ans = comb.nCr(Y1 + X1, Y1);

  FOR(y, 1, H) {
    auto a = comb.nCr(Y1 + y + X1, X1);
    ans += a;

    LL m = (H - y);
    LL n = (W - 1);
    if (n == 0 or m == 0) {
      continue;
    }
    //        FOR(i, 0, m) {
    //            FOR(j, 0, n) {
    //                count += comb.nCr(i + j, i);
    //            }
    //        }
    mint<MOD> count = mint<MOD>(m + 1) * comb.nCr(m + n, n - 1) / n - 1;
    ans += count * a;
  }

  FOR(x, 1, W) {
    auto a = comb.nCr(X1 + x + Y1, Y1);
    ans += a;

    LL m = (H - 1);
    LL n = (W - x);
    if (n == 0 or m == 0) {
      continue;
    }
    //        FOR(i, 0, H - 1) {
    //            FOR(j, 0, W - x) {
    //                int c = comb.nCr(i + j, i);
    //                count += comb.nCr(i + j, i);
    //            }
    //        }
    mint<MOD> count = mint<MOD>(m + 1) * comb.nCr(m + n, n - 1) / n - 1;
    ans += count * a;
  }

  return ans;
}

mint<MOD> check(LL Y1, LL X1, LL Y2, LL X2) {
  mint<MOD> ans = 0;

  FOR(y, Y1, Y2 + 1) {
    FOR(x, X1, X2 + 1) { ans += comb.nCr(y + x, x); }
  }

  return ans;
}

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

  LL Y1, X1, Y2, X2;
  cin >> Y1 >> X1 >> Y2 >> X2;
  print(solve(Y1, X1, Y2, X2).x);

  return 0;
}