Problem p02804 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define all(x) (x).begin(), (x).end()
#define m0(x) memset(x, 0, sizeof(x))

int dx4[4] = {1, 0, -1, 0}, dy4[4] = {0, 1, 0, -1};

template <int M, bool IsPrime = false> class Modulo {
  int n;
  static typename std::enable_if<IsPrime, ll>::type inv(ll a, ll p) {
    return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p);
  }

public:
  Modulo() : n(0) { ; }
  Modulo(int m) : n(m) {
    if (n >= M)
      n %= M;
    else if (n < 0)
      n = (n % M + M) % M;
  }
  Modulo(ll m) {
    if (m >= M)
      m %= M;
    else if (m < 0)
      m = (m % M + M) % M;
    n = m;
  }
  explicit operator int() const { return n; }
  explicit operator ll() const { return n; }
  bool operator==(const Modulo &a) const { return n == a.n; }
  Modulo &operator+=(const Modulo &a) {
    n += a.n;
    if (n >= M)
      n -= M;
    return *this;
  }
  Modulo &operator-=(const Modulo &a) {
    n -= a.n;
    if (n < 0)
      n += M;
    return *this;
  }
  Modulo &operator*=(const Modulo &a) {
    n = (ll(n) * a.n) % M;
    return *this;
  }
  Modulo operator+(const Modulo &a) const {
    Modulo res = *this;
    return res += a;
  }
  Modulo operator-(const Modulo &a) const {
    Modulo res = *this;
    return res -= a;
  }
  Modulo operator-() const { return Modulo(0) - *this; }
  Modulo operator*(const Modulo &a) const {
    Modulo res = *this;
    return res *= a;
  }

  Modulo operator^(ll m) const {
    if (m == 0)
      return Modulo(1);
    const Modulo a = *this;
    Modulo res = (a * a) ^ (m / 2);
    return m % 2 ? res * a : res;
  }
  typename std::enable_if<IsPrime, Modulo>::type
  operator/(const Modulo &a) const {
    return *this * inv(ll(a), M);
  }
  typename std::enable_if<IsPrime, Modulo>::type operator/=(const Modulo &a) {
    return *this *= inv(ll(a), M);
  }

  friend bool is_zero(const Modulo &x) { return int(x) == 0; }
  friend int abs(const Modulo &x) { return int(x); }

  static Modulo fact(int n, bool sw = true) {
    static std::vector<Modulo> v1 = {1}, v2 = {1};
    if (n >= (int)v1.size()) {
      const int from = v1.size(), to = n + 1024;
      v1.reserve(to);
      v2.reserve(to);
      for (int i = from; i < to; ++i) {
        v1.push_back(v1.back() * Modulo<M, true>(i));
        v2.push_back(v2.back() / Modulo<M, true>(i));
      }
    }
    return sw ? v1[n] : v2[n];
  }
  static Modulo comb(int a, int b) {
    if (b < 0 || b > a)
      return 0;
    return Modulo::fact(a, true) * Modulo::fact(b, false) *
           Modulo::fact(a - b, false);
  }
};

using Mod = Modulo<1000000007, true>;

template <typename T> struct Combination {
  vector<T> _fact, _rfact, _inv;

  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for (int i = 1; i <= sz; i++)
      _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for (int i = sz - 1; i >= 0; i--)
      _rfact[i] = _rfact[i + 1] * (i + 1);
    for (int i = 1; i <= sz; i++)
      _inv[i] = _rfact[i] * _fact[i - 1];
  }

  inline T fact(int k) const { return _fact[k]; }

  inline T rfact(int k) const { return _rfact[k]; }

  inline T inv(int k) const { return _inv[k]; }

  T P(int n, int r) const {
    if (r < 0 || n < r)
      return 0;
    return fact(n) * rfact(n - r);
  }

  T C(int p, int q) const {
    if (q < 0 || p < q)
      return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }

  T H(int n, int r) const {
    if (n < 0 || r < 0)
      return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

int main() {
  int n, k;
  cin >> n >> k;
  Mod ans = 0;
  vector<Mod> x(n);
  vector<int> xx(n);
  for (int i = 0; i < n; i++) {
    cin >> xx[i];
  }
  sort(all(xx));
  for (int i = 0; i < n; i++) {
    x[i] = xx[i];
  }

  Combination<Mod> comb(10);

  for (int i = 0; i < n; i++) {
    ans += x[i] * comb.C(i, k - 1);
    ans -= x[i] * comb.C(n - 1 - i, k - 1);
  }

  cout << int(ans) << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define all(x) (x).begin(), (x).end()
#define m0(x) memset(x, 0, sizeof(x))

int dx4[4] = {1, 0, -1, 0}, dy4[4] = {0, 1, 0, -1};

template <int M, bool IsPrime = false> class Modulo {
  int n;
  static typename std::enable_if<IsPrime, ll>::type inv(ll a, ll p) {
    return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p);
  }

public:
  Modulo() : n(0) { ; }
  Modulo(int m) : n(m) {
    if (n >= M)
      n %= M;
    else if (n < 0)
      n = (n % M + M) % M;
  }
  Modulo(ll m) {
    if (m >= M)
      m %= M;
    else if (m < 0)
      m = (m % M + M) % M;
    n = m;
  }
  explicit operator int() const { return n; }
  explicit operator ll() const { return n; }
  bool operator==(const Modulo &a) const { return n == a.n; }
  Modulo &operator+=(const Modulo &a) {
    n += a.n;
    if (n >= M)
      n -= M;
    return *this;
  }
  Modulo &operator-=(const Modulo &a) {
    n -= a.n;
    if (n < 0)
      n += M;
    return *this;
  }
  Modulo &operator*=(const Modulo &a) {
    n = (ll(n) * a.n) % M;
    return *this;
  }
  Modulo operator+(const Modulo &a) const {
    Modulo res = *this;
    return res += a;
  }
  Modulo operator-(const Modulo &a) const {
    Modulo res = *this;
    return res -= a;
  }
  Modulo operator-() const { return Modulo(0) - *this; }
  Modulo operator*(const Modulo &a) const {
    Modulo res = *this;
    return res *= a;
  }

  Modulo operator^(ll m) const {
    if (m == 0)
      return Modulo(1);
    const Modulo a = *this;
    Modulo res = (a * a) ^ (m / 2);
    return m % 2 ? res * a : res;
  }
  typename std::enable_if<IsPrime, Modulo>::type
  operator/(const Modulo &a) const {
    return *this * inv(ll(a), M);
  }
  typename std::enable_if<IsPrime, Modulo>::type operator/=(const Modulo &a) {
    return *this *= inv(ll(a), M);
  }

  friend bool is_zero(const Modulo &x) { return int(x) == 0; }
  friend int abs(const Modulo &x) { return int(x); }

  static Modulo fact(int n, bool sw = true) {
    static std::vector<Modulo> v1 = {1}, v2 = {1};
    if (n >= (int)v1.size()) {
      const int from = v1.size(), to = n + 1024;
      v1.reserve(to);
      v2.reserve(to);
      for (int i = from; i < to; ++i) {
        v1.push_back(v1.back() * Modulo<M, true>(i));
        v2.push_back(v2.back() / Modulo<M, true>(i));
      }
    }
    return sw ? v1[n] : v2[n];
  }
  static Modulo comb(int a, int b) {
    if (b < 0 || b > a)
      return 0;
    return Modulo::fact(a, true) * Modulo::fact(b, false) *
           Modulo::fact(a - b, false);
  }
};

using Mod = Modulo<1000000007, true>;

template <typename T> struct Combination {
  vector<T> _fact, _rfact, _inv;

  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for (int i = 1; i <= sz; i++)
      _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for (int i = sz - 1; i >= 0; i--)
      _rfact[i] = _rfact[i + 1] * (i + 1);
    for (int i = 1; i <= sz; i++)
      _inv[i] = _rfact[i] * _fact[i - 1];
  }

  inline T fact(int k) const { return _fact[k]; }

  inline T rfact(int k) const { return _rfact[k]; }

  inline T inv(int k) const { return _inv[k]; }

  T P(int n, int r) const {
    if (r < 0 || n < r)
      return 0;
    return fact(n) * rfact(n - r);
  }

  T C(int p, int q) const {
    if (q < 0 || p < q)
      return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }

  T H(int n, int r) const {
    if (n < 0 || r < 0)
      return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

int main() {
  int n, k;
  cin >> n >> k;
  Mod ans = 0;
  vector<Mod> x(n);
  vector<int> xx(n);
  for (int i = 0; i < n; i++) {
    cin >> xx[i];
  }
  sort(all(xx));
  for (int i = 0; i < n; i++) {
    x[i] = xx[i];
  }

  Combination<Mod> comb(n + 10);

  for (int i = 0; i < n; i++) {
    ans += x[i] * comb.C(i, k - 1);
    ans -= x[i] * comb.C(n - 1 - i, k - 1);
  }

  cout << int(ans) << endl;

  return 0;
}