Language: C++

//#define NDEBUG
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <utility>
#include <vector>

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

struct rep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
struct revrep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) noexcept {
  if (a < b) {
    return b - a;
  } else {
    return a - b;
  }
}
template <class T> void chmin(T &a, const T &b) noexcept {
  if (b < a) {
    a = b;
  }
}
template <class T> void chmax(T &a, const T &b) noexcept {
  if (a < b) {
    a = b;
  }
}
template <class F> class fix_point : private F {
public:
  explicit constexpr fix_point(F &&f) : F(std::forward<F>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, std::forward<Args>(args)...);
  }
};
template <class F> constexpr decltype(auto) make_fix(F &&f) {
  return fix_point<F>(std::forward<F>(f));
}
template <class T> T scan() {
  T ret;
  std::cin >> ret;
  return ret;
}

} // namespace n91

#include <cstdint>
#include <iostream>

template <std::uint32_t mod> class modint {
  using i64 = std::int64_t;
  using u32 = std::uint32_t;
  using u64 = std::int64_t;

  static_assert(mod < (u32(1) << 31), "mod must be less than 2**31");

public:
  u32 v;

  constexpr modint(const i64 x = 0) noexcept
      : v(x < 0 ? mod - 1 - -(x + 1) % mod : x % mod) {}
  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 modint &operator+=(const modint rhs) noexcept {
    v += rhs.v;
    if (v >= mod)
      v -= mod;
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (v < rhs.v)
      v += mod;
    v -= rhs.v;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    v = u64(v) * rhs.v % mod;
    return *this;
  }
  constexpr modint &operator/=(const modint rhs) noexcept {
    *this *= rhs.pow(mod - 2);
  }
  constexpr modint pow(u32 exp) noexcept {
    modint self(*this), ret(1);
    while (exp != 0) {
      if (exp % 2 != 0)
        ret *= self;
      self *= self;
      exp /= 2;
    }
    return ret;
  }
};

template <std::uint32_t mod>
std::istream &operator>>(std::istream &is, modint<mod> &rhs) {
  std::int64_t v;
  is >> v;
  rhs = modint<mod>(v);
  return is;
}

template <std::uint32_t mod>
std::ostream &operator<<(std::ostream &os, const modint<mod> &rhs) {
  os << rhs.v;
  return os;
}

#include <vector>

namespace n91 {

template <class T> class fact_binom {
  using size_t = std::size_t;

public:
  using value_type = T;

private:
  std::vector<T> fact, inv_fact;

public:
  fact_binom() = default;
  explicit fact_binom(const size_t n) : fact(n + 1), inv_fact(n + 1) {
    fact[0] = 1;
    for (size_t i = 0; i != n; ++i) {
      fact[i + 1] = fact[i] * (i + 1);
    }
    inv_fact[n] = static_cast<value_type>(1) / fact[n];
    for (size_t i = n; i != 0; --i) {
      inv_fact[i - 1] = inv_fact[i] * i;
    }
  }

  T operator()(const size_t n, const size_t r) const {
    return fact[n] * inv_fact[r] * inv_fact[n - r];
  }
};

} // namespace n91

#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>

namespace n91 {

void main_() {
  /*
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  //*/
  using mint = modint<1000000007>;
  const usize n = scan<usize>();
  const usize k = scan<usize>();
  const fact_binom<mint> binom(n);
  std::vector<i32> a(n);
  for (auto &e : a) {
    std::cin >> e;
  }
  std::sort(a.begin(), a.end());
  mint ans = 0;
  for (const usize i : rep(0, n)) {
    if (i >= k - 1)
      ans += binom(i, k - 1) * a[i];
  }
  for (const usize i : rep(0, n)) {
    if (n - i - 1 >= k - 1)
      ans -= binom(n - i - 1, k - 1) * a[i];
  }
  std::cout << ans << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Compiler output:
a.cc: In instantiation of 'constexpr modint<mod>& modint<mod>::operator/=(modint<mod>) [with unsigned int mod = 1000000007]':
a.cc:116:26:   required from 'constexpr modint<mod> modint<mod>::operator/(modint<mod>) const [with unsigned int mod = 1000000007]'
  116 |     return modint(*this) /= rhs;
      |            ~~~~~~~~~~~~~~^~~~~~
a.cc:183:46:   required from 'n91::fact_binom<T>::fact_binom(size_t) [with T = modint<1000000007>; size_t = long unsigned int]'
  183 |     inv_fact[n] = static_cast<value_type>(1) / fact[n];
      |                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
a.cc:211:33:   required from here
  211 |   const fact_binom<mint> binom(n);
      |                                 ^
a.cc:135:21: error: passing 'const modint<1000000007>' as 'this' argument discards qualifiers [-fpermissive]
  135 |     *this *= rhs.pow(mod - 2);
      |              ~~~~~~~^~~~~~~~~
a.cc:137:20: note:   in call to 'constexpr modint<mod> modint<mod>::pow(u32) [with unsigned int mod = 1000000007; u32 = unsigned int]'
  137 |   constexpr modint pow(u32 exp) noexcept {
      |                    ^~~
a.cc:136:3: error: no return statement in 'constexpr' function returning non-void
  135 |     *this *= rhs.pow(mod - 2);
  +++ |+                    return *this;
  136 |   }
      |   ^
a.cc: In member function 'constexpr modint<mod>& modint<mod>::operator/=(modint<mod>) [with unsigned int mod = 1000000007]':
a.cc:135:11: warning: control reaches end of non-void function [-Wreturn-type]
  135 |     *this *= rhs.pow(mod - 2);
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~