Problem p02804 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;

const int mod = 1000000007;
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;
  }
};

// combination mod prime（mintが必要）
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
// nCkを求めて結果をansに格納したい場合、
// combination c(n);
// mint ans = c(n, k);
// cout << ans.x << endl;
struct combination {
  vector<mint> fact, ifact;
  combination(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 operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

int main() {

  int n, k;
  cin >> n >> k;
  vector<ll> a(n);
  rep(i, n) {
    int tmp;
    cin >> tmp;
    a[i] = tmp;
  }
  sort(all(a));
  mint ans = 0;

  combination c(n);
  rep(i, n) { combination c(i); }

  for (int i = k - 1; i < n; i++) {
    mint tmp = c(i, k - 1);
    tmp *= a[i];
    ans += tmp;
  }
  for (int i = 0; i < n - k + 1; i++) {
    mint tmp = c(n - i - 1, k - 1);
    tmp *= a[i];
    ans -= tmp;
  }
  cout << ans.x << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;

const int mod = 1000000007;
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;
  }
};

// combination mod prime（mintが必要）
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
// nCkを求めて結果をansに格納したい場合、
// combination c(n);
// mint ans = c(n, k);
// cout << ans.x << endl;
struct combination {
  vector<mint> fact, ifact;
  combination(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 operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

int main() {

  int n, k;
  cin >> n >> k;
  vector<ll> a(n);
  rep(i, n) {
    int tmp;
    cin >> tmp;
    a[i] = tmp;
  }
  sort(all(a));
  mint ans = 0;

  combination c(n);

  for (int i = k - 1; i < n; i++) {
    mint tmp = c(i, k - 1);
    tmp *= a[i];
    ans += tmp;
  }
  for (int i = 0; i < n - k + 1; i++) {
    mint tmp = c(n - i - 1, k - 1);
    tmp *= a[i];
    ans -= tmp;
  }
  cout << ans.x << endl;
  return 0;
}