инверсии

#include <iostream>
#include <cmath>

using namespace std;

long long merge_series (long long *arr, long long first, long long last_1, long long last_2, long long *dubl)
{
  long long i = first;
  long long j = last_1+1;
  long long c = 0;
  long long sum = 0;

 for (c = first; c <= last_2; c++)
  {
    if (j > last_2)
    dubl[c] = arr[i++];
     else if (i > last_1)
      dubl[c] = arr[j++];
      else if (arr[i] > arr[j])
       {
         dubl[c] = arr[j];
         sum += last_1 - i+1;
         j++;
       }
       else
       {
         dubl[c] = arr[i++];
       }
  }
  //cout << sum << " ";
  return sum;
}

void sort (long long *arr, long long first, long long last, long long *dubl, long long &sum)
{
  int middle = (first + last)/2;
  if (first < middle)
   sort (arr, first, middle, dubl, sum);
  if (middle + 1 < last)
   sort (arr, middle+1, last, dubl, sum);

  sum += merge_series (arr, first, middle, last, dubl);
  //cout << sum << endl;

  for (int i = first; i <= last; i++)
    arr[i] = dubl[i];
 }

int main()
{
  long long n=0;
  cin >> n;
  long long sum = 0;

  long long *arr = new long long[n];
  for (long long i=0; i<n; i++)
   cin >> arr[i];

  long long *dubl = new long long[n];

  sort (arr, 0, n-1, dubl, sum);
  cout << sum << endl;

  //for (int i=0; i < n; i++) cout << arr[i] << " ";
}