Language: C++

#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
typedef tuple<int,int,int> T;
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep(i, n) for (ll i = 0; i < n; i++)
#define debug(x) cout << #x << ' ' << '=' << ' ' << (x) << endl;
#define fs first
#define sc second
#define int long long
#define pb push_back
#define mp make_pair
const int dx[] = {0, -1, 0, 1};
const int dy[] = {1, 0, -1, 0};
const int MOD = (1000000007);
//const int MOD = (998244353);
//const int INF = (1 << 30)-1;
const int INF = (1LL << 60)-1;
const double EPS = (1 >> 30);

template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return 1;} return 0;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return 1;} return 0;}

using Graph = vector<vector<pair<int, int>>>;
using Ki = vector<vector<int>>;
using vec = vector<int>;

#include <cstdint>

template <std::uint_fast64_t Modulus> class modint {
  using u64 = std::uint_fast64_t;

  u64 a;

public:
  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 val() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr bool operator==(const modint rhs) const noexcept {
    return modint(*this).val() == rhs.val();
  }
  modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
using mint = modint<MOD>;
typedef vector<mint> vec;
typedef vector<vector<mint>> mat;

void add(int &a, int b){
  a = ((a+b) % MOD + MOD) % MOD;
}

inline int mul(int a, int b) {
    add(a,0); add(b,0);
    #if !defined(_WIN32) || defined(_WIN64)
      return (int) ((long long) a * b % MOD);
    #endif
      unsigned long long x = (long long) a * b;
      unsigned xh = (unsigned) (x >> 32), xl = (unsigned) x, d, m;
      asm(
        "divl %4; \n\t"
        : "=a" (d), "=d" (m)
        : "d" (xh), "a" (xl), "r" (MOD)
      );
      return m;
}

inline int modpow(int a, long long b) {
  int res = 1;
  while (b > 0) {
    if (b & 1) {
      res = mul(res, a);
      }
      a = mul(a, a);
      b >>= 1;
  }
  return res;
}

inline int inv(int a) {
    a %= MOD;
    if (a < 0) a += MOD;
    int b = MOD, u = 0, v = 1;
    while (a) {
      int t = b / a;
      b -= t * a; swap(a, b);
      u -= t * v; swap(u, v);
    }
    assert(b == 1);
    if (u < 0) u += MOD;
    return u;
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(10);
    //---------------------------------------------
    
    int n,k;
    cin >> n >> k;
 
    vector<int> a(n);
    rep(i,n) cin >> a[i];
 
    sort(a.begin(),a.end());
  
    vec fact(n+1,1);
    rep(i,n) fact[i+1] = fact[i]*(i+1);
 
    mint ans = 0;
    rep(i,n-k+1){
        ans -= fact[n-1-i] / fact[k-1] / fact[n-k-i] * a[i];
    }
 
    reverse(a.begin(),a.end());
    rep(i,n-k+1){
      ans += fact[n-1-i] / fact[k-1] / fact[n-k-i] * a[i];
    }
 
    cout << ans.val << endl;
    
}

Compiler output:
a.cc:87:22: error: conflicting declaration 'typedef class std::__debug::vector<modint<1000000007> > vec'
   87 | typedef vector<mint> vec;
      |                      ^~~
a.cc:29:7: note: previous declaration as 'using vec = class std::__debug::vector<long long int>'
   29 | using vec = vector<int>;
      |       ^~~
a.cc: In function 'int main()':
a.cc:163:10: error: invalid use of non-static member function 'constexpr modint<Modulus>::u64 modint<Modulus>::val() const [with long unsigned int Modulus = 1000000007; u64 = long unsigned int]'
  163 |     cout << ans.val << endl;
      |     ~~~~~^~~~~~~~~~
a.cc:40:17: note: declared here
   40 |   constexpr u64 val() const noexcept { return a; }
      |                 ^~~