Problem p02807 (C++)

Buggy submission (status: Time Limit Exceeded):
/*
Author : N_o_o_B
Created : January 14 2020 18:43:54
*/
#include <bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
*/
#define TRACE
#ifdef TRACE
#define trace(...)                                                             \
  {                                                                            \
    cerr << "[ ";                                                              \
    __trace__(#__VA_ARGS__, __VA_ARGS__);                                      \
  }
#undef endl
template <typename Arg1, typename Arg2>
ostream &operator<<(ostream &out, const pair<Arg1, Arg2> &x) {
  return out << "(" << x.first << "," << x.second << ")";
}

template <typename Arg1>
ostream &operator<<(ostream &out, const vector<Arg1> &a) {
  out << "[";
  for (const auto &x : a)
    out << x << ",";
  return out << "]";
}

template <typename Arg1> ostream &operator<<(ostream &out, const set<Arg1> &a) {
  out << "[";
  for (const auto &x : a)
    out << x << ",";
  return out << "]";
}

template <typename Arg1, typename Arg2>
ostream &operator<<(ostream &out, const map<Arg1, Arg2> &a) {
  out << "[";
  for (const auto &x : a)
    out << x << ",";
  return out << "]";
}

template <typename Arg1> void __trace__(const string name, Arg1 &&arg1) {
  cerr << name << " : " << arg1 << " ] " << endl;
}

template <typename Arg1, typename... Args>
void __trace__(const string names, Arg1 &&arg1, Args &&...args) {
  const string name = names.substr(0, names.find(','));
  cerr << name << " : " << arg1 << " | ";
  __trace__(names.substr(1 + (int)name.size()), args...);
}
#else
#define trace(args...)
#endif

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pair<int, int>> vii;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<pair<ll, ll>> vll;
typedef vector<vl> vvl;
// typedef tree<pii, null_type, less<pii>, rb_tree_tag,
// tree_order_statistics_node_update> oset;
#define fori(i, n) for (int i = 0; i < n; i++)
#define rep(i, a, b) for (int i = a; i <= b; i++)
#define repd(i, a, b) for (int i = a; i >= b; i--)
#define ford(i, n) for (int i = n - 1; i >= 0; i--)
#define trav(x, a) for (auto &x : a)
#define all(x) x.begin(), x.end()
#define pb push_back
#define endl '\n'
#define sz(a) (int)a.size()
#define fi first
#define se second
clock_t time_p = clock();
void time_taken() {
  time_p = clock() - time_p;
  cerr << "Time Taken : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}
// const ll mod=998244353;
const ll mod = 1e9 + 7;
const ll INF = 1e18;

// credits ---> ksun48

ll modinv(ll a, ll m) {
  assert(m > 0);
  if (m == 1)
    return 0;
  a %= m;
  if (a < 0)
    a += m;
  assert(a != 0);
  if (a == 1)
    return 1;
  return m - modinv(m, a) * m / a;
}

template <int MOD_> struct modnum {
private:
  int v;

public:
  static const int MOD = MOD_;

  modnum() : v(0) {}
  modnum(ll v_) : v(int(v_ % MOD)) {
    if (v < 0)
      v += MOD;
  }
  explicit operator int() const { return v; }
  friend bool operator==(const modnum &a, const modnum &b) {
    return a.v == b.v;
  }
  friend bool operator!=(const modnum &a, const modnum &b) {
    return a.v != b.v;
  }

  modnum operator~() const {
    modnum res;
    res.v = modinv(v, MOD);
    return res;
  }

  modnum &operator+=(const modnum &o) {
    v += o.v;
    if (v >= MOD)
      v -= MOD;
    return *this;
  }
  modnum &operator-=(const modnum &o) {
    v -= o.v;
    if (v < 0)
      v += MOD;
    return *this;
  }
  modnum &operator*=(const modnum &o) {
    v = int(ll(v) * ll(o.v) % MOD);
    return *this;
  }
  modnum &operator/=(const modnum &o) { return *this *= (~o); }

  friend modnum operator+(const modnum &a, const modnum &b) {
    return modnum(a) += b;
  }
  friend modnum operator-(const modnum &a, const modnum &b) {
    return modnum(a) -= b;
  }
  friend modnum operator*(const modnum &a, const modnum &b) {
    return modnum(a) *= b;
  }
  friend modnum operator/(const modnum &a, const modnum &b) {
    return modnum(a) /= b;
  }
};

using num = modnum<int(1e9) + 7>;

vector<num> fact;
vector<num> ifact;

void init() {
  fact = {1};
  for (int i = 1; i < 100000; i++)
    fact.push_back(i * fact[i - 1]);
  for (num x : fact)
    ifact.push_back(1 / x);
}

num ncr(int n, int k) {
  if (k < 0 || k > n)
    return 0;
  return fact[n] * ifact[k] * ifact[n - k];
}

num powmod(num x, int a) {
  if (a == 0)
    return 1;
  if (a & 1)
    return x * powmod(x, a - 1);
  return powmod(x * x, a / 2);
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cout.precision(12);
  cout << fixed;
  init();
  int n;
  cin >> n;
  vi x(n);
  fori(i, n) cin >> x[i];
  num ans = x[n - 1] - x[n - 2];
  ford(i, n - 2) {
    rep(j, i + 1, n - 2) {
      ans += num(1) / (j - i + 1) * (x[j] - x[i]) / (j - i);
    }
    ans += num(1) / (n - 1 - i) * (x[n - 1] - x[i]);
  }
  ans *= fact[n - 1];
  cout << int(ans) << endl;

  time_taken();
  return 0;
}

Fixed submission:
/*
Author : N_o_o_B
Created : January 14 2020 18:43:54
*/
#include <bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
*/
#define TRACE
#ifdef TRACE
#define trace(...)                                                             \
  {                                                                            \
    cerr << "[ ";                                                              \
    __trace__(#__VA_ARGS__, __VA_ARGS__);                                      \
  }
#undef endl
template <typename Arg1, typename Arg2>
ostream &operator<<(ostream &out, const pair<Arg1, Arg2> &x) {
  return out << "(" << x.first << "," << x.second << ")";
}

template <typename Arg1>
ostream &operator<<(ostream &out, const vector<Arg1> &a) {
  out << "[";
  for (const auto &x : a)
    out << x << ",";
  return out << "]";
}

template <typename Arg1> ostream &operator<<(ostream &out, const set<Arg1> &a) {
  out << "[";
  for (const auto &x : a)
    out << x << ",";
  return out << "]";
}

template <typename Arg1, typename Arg2>
ostream &operator<<(ostream &out, const map<Arg1, Arg2> &a) {
  out << "[";
  for (const auto &x : a)
    out << x << ",";
  return out << "]";
}

template <typename Arg1> void __trace__(const string name, Arg1 &&arg1) {
  cerr << name << " : " << arg1 << " ] " << endl;
}

template <typename Arg1, typename... Args>
void __trace__(const string names, Arg1 &&arg1, Args &&...args) {
  const string name = names.substr(0, names.find(','));
  cerr << name << " : " << arg1 << " | ";
  __trace__(names.substr(1 + (int)name.size()), args...);
}
#else
#define trace(args...)
#endif

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pair<int, int>> vii;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<pair<ll, ll>> vll;
typedef vector<vl> vvl;
// typedef tree<pii, null_type, less<pii>, rb_tree_tag,
// tree_order_statistics_node_update> oset;
#define fori(i, n) for (int i = 0; i < n; i++)
#define rep(i, a, b) for (int i = a; i <= b; i++)
#define repd(i, a, b) for (int i = a; i >= b; i--)
#define ford(i, n) for (int i = n - 1; i >= 0; i--)
#define trav(x, a) for (auto &x : a)
#define all(x) x.begin(), x.end()
#define pb push_back
#define endl '\n'
#define sz(a) (int)a.size()
#define fi first
#define se second
clock_t time_p = clock();
void time_taken() {
  time_p = clock() - time_p;
  cerr << "Time Taken : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}
// const ll mod=998244353;
const ll mod = 1e9 + 7;
const ll INF = 1e18;

// credits ---> ksun48

ll modinv(ll a, ll m) {
  assert(m > 0);
  if (m == 1)
    return 0;
  a %= m;
  if (a < 0)
    a += m;
  assert(a != 0);
  if (a == 1)
    return 1;
  return m - modinv(m, a) * m / a;
}

template <int MOD_> struct modnum {
private:
  int v;

public:
  static const int MOD = MOD_;

  modnum() : v(0) {}
  modnum(ll v_) : v(int(v_ % MOD)) {
    if (v < 0)
      v += MOD;
  }
  explicit operator int() const { return v; }
  friend bool operator==(const modnum &a, const modnum &b) {
    return a.v == b.v;
  }
  friend bool operator!=(const modnum &a, const modnum &b) {
    return a.v != b.v;
  }

  modnum operator~() const {
    modnum res;
    res.v = modinv(v, MOD);
    return res;
  }

  modnum &operator+=(const modnum &o) {
    v += o.v;
    if (v >= MOD)
      v -= MOD;
    return *this;
  }
  modnum &operator-=(const modnum &o) {
    v -= o.v;
    if (v < 0)
      v += MOD;
    return *this;
  }
  modnum &operator*=(const modnum &o) {
    v = int(ll(v) * ll(o.v) % MOD);
    return *this;
  }
  modnum &operator/=(const modnum &o) { return *this *= (~o); }

  friend modnum operator+(const modnum &a, const modnum &b) {
    return modnum(a) += b;
  }
  friend modnum operator-(const modnum &a, const modnum &b) {
    return modnum(a) -= b;
  }
  friend modnum operator*(const modnum &a, const modnum &b) {
    return modnum(a) *= b;
  }
  friend modnum operator/(const modnum &a, const modnum &b) {
    return modnum(a) /= b;
  }
};

using num = modnum<int(1e9) + 7>;

vector<num> fact;
vector<num> ifact;

void init() {
  fact = {1};
  for (int i = 1; i < 100000; i++)
    fact.push_back(i * fact[i - 1]);
  for (num x : fact)
    ifact.push_back(1 / x);
}

num ncr(int n, int k) {
  if (k < 0 || k > n)
    return 0;
  return fact[n] * ifact[k] * ifact[n - k];
}

num powmod(num x, int a) {
  if (a == 0)
    return 1;
  if (a & 1)
    return x * powmod(x, a - 1);
  return powmod(x * x, a / 2);
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cout.precision(12);
  cout << fixed;
  init();
  int n;
  cin >> n;
  vi x(n);
  fori(i, n) cin >> x[i];
  num ans = 0;
  vector<num> foo(n);
  foo[0] = 0;
  rep(i, 1, n - 1) { foo[i] = foo[i - 1] + (num(1) / i) / (i + 1); }
  ford(i, n - 1) {
    ans += (foo[i] - foo[n - 2 - i]) * x[i];
    ans += num(1) / (n - 1 - i) * (x[n - 1] - x[i]);
  }
  ans *= fact[n - 1];
  cout << int(ans) << endl;

  time_taken();
  return 0;
}