Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
#define repo(i, n) for (int i = 1; i <= n; i++)
#define ssort(a) sort(a.begin(), a.end())
#define INF 1001001001
#define INFll 100100100100100
// debug用
#define PrintVec(x)                                                            \
  for (auto elementPrintVec : x) {                                             \
    cout << elementPrintVec << " ";                                            \
  }                                                                            \
  cout << endl;

using namespace std;
using ull = unsigned long long;
using ll = long long;
using P = pair<int, int>;
const int mod = 1000000007;

#include <iostream>
using namespace std;
/*
1≤k≤n≤10^7
p は素数 (上の実装ではさらに p>n を仮定している)
*/

const int MAX = 510000; // 階乗の配列数
const int MOD = 1000000007;

long long 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
long long 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;
}

// mod. m での a の逆元 a^{-1} を計算する
long long modinv(long long a, long long m) {
  long long b = m, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}

// a^n mod を計算する
long long modpow(long long a, long long n, long long mod) {
  if (a < n)
    return 0;
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

int main() {
  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  COMinit();
  ll ans = 0;
  for (int x = r1; x <= r2; x++) {
    ans += COM((ll)(x + 1 + c2), (ll)c2) - COM((ll)(x + c1), (ll)(x + 1));
    ans += mod;
    ans %= mod;
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
#define repo(i, n) for (int i = 1; i <= n; i++)
#define ssort(a) sort(a.begin(), a.end())
#define INF 1001001001
#define INFll 100100100100100
// debug用
#define PrintVec(x)                                                            \
  for (auto elementPrintVec : x) {                                             \
    cout << elementPrintVec << " ";                                            \
  }                                                                            \
  cout << endl;

using namespace std;
using ull = unsigned long long;
using ll = long long;
using P = pair<int, int>;
const int mod = 1000000007;

#include <iostream>
using namespace std;
/*
1≤k≤n≤10^7
p は素数 (上の実装ではさらに p>n を仮定している)
*/

const int MAX = 2000100; // 階乗の配列数
const int MOD = 1000000007;

long long 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
long long 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;
}

// mod. m での a の逆元 a^{-1} を計算する
long long modinv(long long a, long long m) {
  long long b = m, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}

// a^n mod を計算する
long long modpow(long long a, long long n, long long mod) {
  if (a < n)
    return 0;
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

int main() {
  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  COMinit();
  ll ans = 0;
  for (int x = r1; x <= r2; x++) {
    ans += COM((ll)(x + 1 + c2), (ll)c2) - COM((ll)(x + c1), (ll)(x + 1));
    ans += mod;
    ans %= mod;
  }
  cout << ans << endl;
  return 0;
}