Problem p02807 (C++)

Buggy submission (status: Time Limit Exceeded):
#define DEBUG 1
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstring>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vll>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
using tll = tuple<ll, ll, ll>;
using vtll = vector<tll>;
using vvtll = vector<vtll>;
#define all(v) (v).begin(), (v).end()
#define for1(i, n) for (ll i = 0; i < (n); i++)
#define for2(i, m, n) for (ll i = (m); i < (n); i++)
#define for3(i, m, n, d) for (ll i = (m); i < (n); i += (d))
#define rfor2(i, m, n) for (ll i = (m); i > (n); i--)
#define rfor3(i, m, n, d) for (ll i = (m); i > (n); i += (d))
#define PI 3.1415926535897932384626433832795028841971693993751L
#define INF 1111111111111111111LL
#define print(...) print_1(__VA_ARGS__)
#define in(...) in_1(__VA_ARGS__)
#if DEBUG
#define dump(...) dump_1(#__VA_ARGS__, __VA_ARGS__)
#define dumpa(...) dumpa_1(#__VA_ARGS__, __VA_ARGS__)
#else
#define dump(...)
#define dumpa(...)
#endif
template <typename Head> void dump_1(const char *str, Head &&h) {
  cerr << str << ": " << h << '\n';
}
template <typename Head, typename... Tail>
void dump_1(const char *str, Head &&h, Tail &&...t) {
  while (*str != ',') {
    cerr << *str++;
  }
  cerr << ": " << h << ' ';
  dump_1(str + 1, t...);
}
template <typename T>
void dumpa_1(const char *str, const T v[], const ll size) {
  while (*str != ',') {
    cerr << *str++;
  }
  cerr << ": ";
  for1(i, size) {
    if (i != 0) {
      cerr << ' ';
    }
    cerr << v[i];
  }
  cerr << '\n';
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &v) {
  os << v.first << ' ' << v.second;
  return os;
}
template <typename T1, typename T2, typename T3>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3> &v) {
  os << get<0>(v) << ' ' << get<1>(v) << ' ' << get<2>(v);
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ' ';
    }
    os << *it;
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ' ';
    }
    os << *it;
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &v) {
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ' ';
    }
    os << *it;
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &v) {
  os << '{';
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ", ";
    }
    os << it->first << ':' << it->second;
  }
  os << '}';
  return os;
}
void Yes(void) { cout << "Yes\n"; }
void No(void) { cout << "No\n"; }
void YES(void) { cout << "YES\n"; }
void NO(void) { cout << "NO\n"; }
template <typename T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> void vin(vector<T> &v, ll len) {
  for1(i, len) { cin >> v[i]; }
}
template <typename Head> void in_1(Head &h) { cin >> h; }
template <typename Head, typename... Tail> void in_1(Head &h, Tail &...t) {
  cin >> h;
  in_1(t...);
}
template <typename Head> void print_1(Head &&h) { cout << h << '\n'; }
template <typename Head, typename... Tail> void print_1(Head &&h, Tail &&...t) {
  cout << h << ' ';
  print_1(t...);
}
//---------------------------------------------------------
const ll mod = 1000000007LL; // 10**9 + 7
struct mint {
  ll x; // typedef long long ll;
  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;
  }
  friend ostream &operator<<(ostream &os, const mint &v) {
    os << v.x;
    return os;
  }
};
//---------------------------------------------------------
struct mintcomb {
  vector<mint> fact, ifact;
  mintcomb(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) {
      fact[i] = fact[i - 1] * i;
    }
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) {
      ifact[i - 1] = ifact[i] * i;
    }
  }
  mint permutation(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[n - k];
  }
  mint combination(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};
//---------------------------------------------------------
mint dp[2][100005];
void solve() {
  ll N;
  in(N);
  vll X(N);
  vin(X, N);
  mint ans = 0;
  mint two = 2;
  mint a = two.pow(N - 2);
  mint r = two.pow(N - 3);
  vector<mint> A(N + 5);
  vector<mint> B(N + 5);
  mintcomb comb(N);
  ll i0 = 1;
  ll i1 = 0;
  dp[i1][1] = 2;
  dp[i1][2] = 3;
  for2(i, 3, N) {
    i0 ^= 1;
    i1 ^= 1;
    dp[i1][1] = comb.fact[i];
    for2(j, 2, i + 1) {
      // dump(i0, j - 1, dp[i0][j - 1], (j), dp[i0][j], (i - j),
      //     comb.fact[i - 1]);
      dp[i1][j] = dp[i0][j - 1] * (j) + dp[i0][j] * (i - j) + comb.fact[i - 1];
    }
    // dump(i);
    // dumpa(dp[i1], N + 1);
  }

  for1(i, N - 1) { ans += dp[i1][i + 1] * (X[i + 1] - X[i]); }
  // dump(ans);
  // mint b = comb.permutation(N - 2, N - 2);
  // ans *= b;
  print(ans);
}
//---------------------------------------------------------
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(16);
  cerr << fixed << setprecision(16);
  solve();
}


Fixed submission:
#define DEBUG 1
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstring>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vll>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
using tll = tuple<ll, ll, ll>;
using vtll = vector<tll>;
using vvtll = vector<vtll>;
#define all(v) (v).begin(), (v).end()
#define for1(i, n) for (ll i = 0; i < (n); i++)
#define for2(i, m, n) for (ll i = (m); i < (n); i++)
#define for3(i, m, n, d) for (ll i = (m); i < (n); i += (d))
#define rfor2(i, m, n) for (ll i = (m); i > (n); i--)
#define rfor3(i, m, n, d) for (ll i = (m); i > (n); i += (d))
#define PI 3.1415926535897932384626433832795028841971693993751L
#define INF 1111111111111111111LL
#define print(...) print_1(__VA_ARGS__)
#define in(...) in_1(__VA_ARGS__)
#if DEBUG
#define dump(...) dump_1(#__VA_ARGS__, __VA_ARGS__)
#define dumpa(...) dumpa_1(#__VA_ARGS__, __VA_ARGS__)
#else
#define dump(...)
#define dumpa(...)
#endif
template <typename Head> void dump_1(const char *str, Head &&h) {
  cerr << str << ": " << h << '\n';
}
template <typename Head, typename... Tail>
void dump_1(const char *str, Head &&h, Tail &&...t) {
  while (*str != ',') {
    cerr << *str++;
  }
  cerr << ": " << h << ' ';
  dump_1(str + 1, t...);
}
template <typename T>
void dumpa_1(const char *str, const T v[], const ll size) {
  while (*str != ',') {
    cerr << *str++;
  }
  cerr << ": ";
  for1(i, size) {
    if (i != 0) {
      cerr << ' ';
    }
    cerr << v[i];
  }
  cerr << '\n';
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &v) {
  os << v.first << ' ' << v.second;
  return os;
}
template <typename T1, typename T2, typename T3>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3> &v) {
  os << get<0>(v) << ' ' << get<1>(v) << ' ' << get<2>(v);
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ' ';
    }
    os << *it;
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ' ';
    }
    os << *it;
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &v) {
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ' ';
    }
    os << *it;
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &v) {
  os << '{';
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin()) {
      os << ", ";
    }
    os << it->first << ':' << it->second;
  }
  os << '}';
  return os;
}
void Yes(void) { cout << "Yes\n"; }
void No(void) { cout << "No\n"; }
void YES(void) { cout << "YES\n"; }
void NO(void) { cout << "NO\n"; }
template <typename T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> void vin(vector<T> &v, ll len) {
  for1(i, len) { cin >> v[i]; }
}
template <typename Head> void in_1(Head &h) { cin >> h; }
template <typename Head, typename... Tail> void in_1(Head &h, Tail &...t) {
  cin >> h;
  in_1(t...);
}
template <typename Head> void print_1(Head &&h) { cout << h << '\n'; }
template <typename Head, typename... Tail> void print_1(Head &&h, Tail &&...t) {
  cout << h << ' ';
  print_1(t...);
}
//---------------------------------------------------------
const ll mod = 1000000007LL; // 10**9 + 7
struct mint {
  ll x; // typedef long long ll;
  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;
  }
  friend ostream &operator<<(ostream &os, const mint &v) {
    os << v.x;
    return os;
  }
};
//---------------------------------------------------------
struct mintcomb {
  vector<mint> fact, ifact;
  mintcomb(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) {
      fact[i] = fact[i - 1] * i;
    }
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) {
      ifact[i - 1] = ifact[i] * i;
    }
  }
  mint permutation(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[n - k];
  }
  mint combination(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};
//---------------------------------------------------------
mint dp[2][100005];
void solve() {
  ll N;
  in(N);
  vll X(N);
  vin(X, N);
  mint ans = 0;
  mint two = 2;
  mint a = two.pow(N - 2);
  mint r = two.pow(N - 3);
  vector<mint> A(N + 5);
  vector<mint> B(N + 5);
  mintcomb comb(N);
  ll i0 = 1;
  ll i1 = 0;
  dp[i1][1] = 2;
  dp[i1][2] = 3;
  /*
  for2 (i, 3, N) {
      i0 ^= 1;
      i1 ^= 1;
      dp[i1][1] = comb.fact[i];
      for2 (j, 2, i + 1) {
          // dump(i0, j - 1, dp[i0][j - 1], (j), dp[i0][j], (i - j),
          //     comb.fact[i - 1]);
          dp[i1][j] =
              dp[i0][j - 1] * (j) + dp[i0][j] * (i - j) + comb.fact[i - 1];
      }
      dump(i);
      dumpa(dp[i1], N + 1);
      for1 (i, N) {
          cout << dp[i1][1] / (dp[i1][i + 1] - dp[i1][i]) << " ";
      }
      cout << endl;
  }
  */
  dp[i1][1] = comb.fact[N - 1];
  for2(i, 2, N) { dp[i1][i] = dp[i1][i - 1] + dp[i1][1] / i; }
  // dumpa(dp[i1], N + 1);

  for1(i, N - 1) { ans += dp[i1][i + 1] * (X[i + 1] - X[i]); }
  // dump(ans);
  // mint b = comb.permutation(N - 2, N - 2);
  // ans *= b;
  print(ans);
}
//---------------------------------------------------------
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(16);
  cerr << fixed << setprecision(16);
  solve();
}