Problem p02804 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define eb emplace_back
#define emp emplace
#define ins insert
#define mp make_pair
#define mt make_tuple
#define sz(s) (int)s.size()
#define forp(i, a, b) for (int i = a; i <= b; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define ren(i, n) for (int i = n - 1; i >= 0; i--)
#define forn(i, a, b) for (int i = a; i >= b; i--)
#define w(t) while (t)
#define sc(a) scanf("%d", &a);
#define on cout << "\n"
#define os cout << " "
#define o2(a, b) cout << a << " " << b
#define o(a) cout << a
#define ppr(a) cout << a.first << " " << a.second
#define bitcount __builtin_popcount
#define gcd __gcd
#define all(v) v.begin(), v.end()
#define mem(n, m) memset(n, m, sizeof(n))
#define pii pair<int, int>
#define tiii tuple<int, int, int>
#define pll pair<long long, long long>
#define sii set<int>
#define sll set<long long>
#define vii vector<int>
#define vll vector<long long>
#define mii map<int, int>
#define mll map<long long, long long>
#define lob lower_bound
#define upb upper_bound
#define ret return
#define present(s, x) (s.find(x) != s.end())
#define cpresent(s, x) (find(all(s), x) != s.end())
#define ford(container, it)                                                    \
  for (auto it = container.begin(); it != container.end(); it++)
#define fors(container, it, a, b) for (auto it = a; it != b; it++)
#define boost                                                                  \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define MOD 1000000007
#define EPSILON 1e-9
#define PI acos(-1)
#define int long long
#define inf 1e9
#define debug(a)                                                               \
  cerr << #a << ": ";                                                          \
  for (auto i : a)                                                             \
    cerr << i << " ";                                                          \
  cerr << '\n';
#define trace(a) cerr << #a << ": " << a << "\n"
#define ordered_set                                                            \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>

typedef long long ll;
typedef long double ldo;
typedef double db;
using namespace std;
auto start = std::chrono::system_clock::now();
inline void skj() {
  std::chrono::time_point<std::chrono::system_clock> end;
  end = std::chrono::system_clock::now();
  std::chrono::duration<double> elapsed_seconds = end - start;
  std::time_t end_time = std::chrono::system_clock::to_time_t(end);
  cerr << "Time taken " << elapsed_seconds.count() * 1000 << "\n";
}

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
// uniform_int_distribution<int> ud(1, 100); use this for random number
// generator

// custom hash for unordered map
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
inline int binexp(int a, int b, int m) {
  if (a == 0) {
    ret 0;
  }
  int res = 1;
  a %= m;
  while (b) {
    if (b & 1)
      res = (res * 1ll * a) % m;
    a = (a * 1ll * a) % m;
    b >>= 1;
  }
  return res;
}
const int N = 1e5 + 5;
int fact[N], ifact[N];
inline int ncr(int n, int r) {
  int ans = fact[n];
  ans = (ans * ifact[n - r]) % MOD;
  ans = (ans * ifact[r]) % MOD;
  ret ans;
}
inline void solve_me_senpai() {
  int n, k;
  cin >> n >> k;
  fact[0] = 1;
  forp(i, 1, n) { fact[i] = (fact[i - 1] * i) % MOD; }
  ifact[n] = binexp(fact[n], MOD - 2, MOD);
  forn(i, n - 1, 0) { ifact[i] = (ifact[i + 1] * (i + 1)) % MOD; }
  vii arr(n);
  rep(i, n) cin >> arr[i];
  sort(all(arr));
  int ans = 0;
  forp(i, k - 1, n - 1) {
    ans = (ans + (ncr(i, k - 1) * (MOD + arr[i]) % MOD) % MOD) % MOD;
  }
  // o(ans);on;
  rep(i, n - k + 1) {
    ans = (ans +
           (MOD - (ncr(n - i - 1, k - 1) * (MOD + arr[i]) % MOD) % MOD) % MOD) %
          MOD;
  }
  o(ans);
}
signed main() {
  boost
#ifndef ONLINE_JUDGE
      freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int t = 1;
  // cin >> t;
  int a = 1;
  while (t--) {
    // cout << "Case #" << a  << ": ";
    solve_me_senpai();
    a++;
  }
  // skj();
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define eb emplace_back
#define emp emplace
#define ins insert
#define mp make_pair
#define mt make_tuple
#define sz(s) (int)s.size()
#define forp(i, a, b) for (int i = a; i <= b; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define ren(i, n) for (int i = n - 1; i >= 0; i--)
#define forn(i, a, b) for (int i = a; i >= b; i--)
#define w(t) while (t)
#define sc(a) scanf("%d", &a);
#define on cout << "\n"
#define os cout << " "
#define o2(a, b) cout << a << " " << b
#define o(a) cout << a
#define ppr(a) cout << a.first << " " << a.second
#define bitcount __builtin_popcount
#define gcd __gcd
#define all(v) v.begin(), v.end()
#define mem(n, m) memset(n, m, sizeof(n))
#define pii pair<int, int>
#define tiii tuple<int, int, int>
#define pll pair<long long, long long>
#define sii set<int>
#define sll set<long long>
#define vii vector<int>
#define vll vector<long long>
#define mii map<int, int>
#define mll map<long long, long long>
#define lob lower_bound
#define upb upper_bound
#define ret return
#define present(s, x) (s.find(x) != s.end())
#define cpresent(s, x) (find(all(s), x) != s.end())
#define ford(container, it)                                                    \
  for (auto it = container.begin(); it != container.end(); it++)
#define fors(container, it, a, b) for (auto it = a; it != b; it++)
#define boost                                                                  \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define MOD 1000000007
#define EPSILON 1e-9
#define PI acos(-1)
#define int long long
#define inf 1e9
#define debug(a)                                                               \
  cerr << #a << ": ";                                                          \
  for (auto i : a)                                                             \
    cerr << i << " ";                                                          \
  cerr << '\n';
#define trace(a) cerr << #a << ": " << a << "\n"
#define ordered_set                                                            \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>

typedef long long ll;
typedef long double ldo;
typedef double db;
using namespace std;
auto start = std::chrono::system_clock::now();
inline void skj() {
  std::chrono::time_point<std::chrono::system_clock> end;
  end = std::chrono::system_clock::now();
  std::chrono::duration<double> elapsed_seconds = end - start;
  std::time_t end_time = std::chrono::system_clock::to_time_t(end);
  cerr << "Time taken " << elapsed_seconds.count() * 1000 << "\n";
}

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
// uniform_int_distribution<int> ud(1, 100); use this for random number
// generator

// custom hash for unordered map
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
inline int binexp(int a, int b, int m) {
  if (a == 0) {
    ret 0;
  }
  int res = 1;
  a %= m;
  while (b) {
    if (b & 1)
      res = (res * 1ll * a) % m;
    a = (a * 1ll * a) % m;
    b >>= 1;
  }
  return res;
}
const int N = 1e5 + 5;
int fact[N], ifact[N];
inline int ncr(int n, int r) {
  int ans = fact[n];
  ans = (ans * ifact[n - r]) % MOD;
  ans = (ans * ifact[r]) % MOD;
  ret ans;
}
inline void solve_me_senpai() {
  int n, k;
  cin >> n >> k;
  fact[0] = 1;
  forp(i, 1, n) { fact[i] = (fact[i - 1] * i) % MOD; }
  ifact[n] = binexp(fact[n], MOD - 2, MOD);
  forn(i, n - 1, 0) { ifact[i] = (ifact[i + 1] * (i + 1)) % MOD; }
  vii arr(n);
  rep(i, n) cin >> arr[i];
  sort(all(arr));
  int ans = 0;
  forp(i, k - 1, n - 1) {
    ans = (ans + (ncr(i, k - 1) * (MOD + arr[i]) % MOD) % MOD) % MOD;
  }
  // o(ans);on;
  rep(i, n - k + 1) {
    ans = (ans +
           (MOD - (ncr(n - i - 1, k - 1) * (MOD + arr[i]) % MOD) % MOD) % MOD) %
          MOD;
  }
  o(ans);
}
signed main() {
  boost int t = 1;
  // cin >> t;
  int a = 1;
  while (t--) {
    // cout << "Case #" << a  << ": ";
    solve_me_senpai();
    a++;
  }
  // skj();
  return 0;
}