Problem p02807 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T, class U> using P = pair<T, U>;
template <class T> using vec = vector<T>;
template <class T> using vvec = vector<vec<T>>;

constexpr ll mod = 1e9 + 7;
struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % 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(ll 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;
  }
};

class combination {
private:
  vector<mint> fact, inv, finv;

public:
  combination(int N) {
    fact = inv = finv = vector<mint>(N + 1);
    fact[0] = fact[1] = 1;
    inv[0] = inv[1] = 1;
    finv[0] = finv[1] = 1;
    for (ll i = 2; i <= N; i++) {
      fact[i] = fact[i - 1] * i;
      inv[i] = (mint)mod - inv[mod % i] * (mod / i);
      finv[i] = finv[i - 1] * inv[i];
    }
  }
  mint f(int i) { return fact[i]; }
  mint comb(int n, int k) {
    if (n < k)
      return 0;
    if (n < 0 || k < 0)
      return 0;
    return fact[n] * finv[k] * finv[n - k];
  }
  mint hcomb(int n, int k) {
    if (n == 0 && k == 0)
      return 1;
    return comb(n + k - 1, k);
  }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N;
  cin >> N;
  assert(N <= 2000);
  vec<ll> X(N);
  combination C(N);
  for (int i = 0; i < N; i++)
    cin >> X[i];
  mint ans = 0;
  vec<mint> S(N + 1, 0);
  for (int i = 1; i < N; i++) {
    mint a = i;
    S[i] = S[i - 1] + a.inv();
    ans += S[i] * (X[i] - X[i - 1]);
  }
  ans *= C.f(N - 1);
  cout << ans.x << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T, class U> using P = pair<T, U>;
template <class T> using vec = vector<T>;
template <class T> using vvec = vector<vec<T>>;

constexpr ll mod = 1e9 + 7;
struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % 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(ll 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;
  }
};

class combination {
private:
  vector<mint> fact, inv, finv;

public:
  combination(int N) {
    fact = inv = finv = vector<mint>(N + 1);
    fact[0] = fact[1] = 1;
    inv[0] = inv[1] = 1;
    finv[0] = finv[1] = 1;
    for (ll i = 2; i <= N; i++) {
      fact[i] = fact[i - 1] * i;
      inv[i] = (mint)mod - inv[mod % i] * (mod / i);
      finv[i] = finv[i - 1] * inv[i];
    }
  }
  mint f(int i) { return fact[i]; }
  mint comb(int n, int k) {
    if (n < k)
      return 0;
    if (n < 0 || k < 0)
      return 0;
    return fact[n] * finv[k] * finv[n - k];
  }
  mint hcomb(int n, int k) {
    if (n == 0 && k == 0)
      return 1;
    return comb(n + k - 1, k);
  }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N;
  cin >> N;
  vec<ll> X(N);
  combination C(N);
  for (int i = 0; i < N; i++)
    cin >> X[i];
  mint ans = 0;
  vec<mint> S(N + 1, 0);
  for (int i = 1; i < N; i++) {
    mint a = i;
    S[i] = S[i - 1] + a.inv();
    ans += S[i] * (X[i] - X[i - 1]);
  }
  ans *= C.f(N - 1);
  cout << ans.x << endl;
}