C++ OpenMP merge sort

#include "iostream"
#include "omp.h"
#include "vector"
#include "time.h"

using namespace std;

vector<long> merge(const vector<long>& left, const vector<long>& right)
{
    vector<long> result;
    unsigned left_it = 0, right_it = 0;

    while(left_it < left.size() && right_it < right.size())
    {
        if(left[left_it] < right[right_it])
        {
            result.push_back(left[left_it]);
            left_it++;
        }
        else
        {
            result.push_back(right[right_it]);
            right_it++;
        }
    }

    // Push the remaining data from both vectors onto the resultant
    while(left_it < left.size())
    {
        result.push_back(left[left_it]);
        left_it++;
    }

    while(right_it < right.size())
    {
        result.push_back(right[right_it]);
        right_it++;
    }

    return result;
}

vector<long> mergesort(vector<long>& vec, int threads)
{
    // Termination condition: List is completely sorted if it
    // only contains a single element.
    if(vec.size() == 1)
    {
        return vec;
    }

    // Determine the location of the middle element in the vector
    std::vector<long>::iterator middle = vec.begin() + (vec.size() / 2);

    vector<long> left(vec.begin(), middle);
    vector<long> right(middle, vec.end());

    // Perform a merge sort on the two smaller vectors

    if (threads > 1)
    {
      #pragma omp parallel sections
      {
        #pragma omp section
        {
          left = mergesort(left, threads/2);
        }
        #pragma omp section
        {
          right = mergesort(right, threads - threads/2);
        }
      }
    }
    else
    {
      left = mergesort(left, 1);
      right = mergesort(right, 1);
    }

    return merge(left, right);
}

int main(){
  int size;
  omp_set_nested(1);
  cout<<omp_get_num_threads()<<"\n";
  cout<<"Input size of array\n";
  cin>>size;
  vector<long> v(size);
  srand(time(NULL));
  cout<<"Initializing array...\n";
  for (long i=0; i<size; ++i){
    v[i] = rand()%100000;
  }
  /*for(int i = 0;i<size;++i){
      cout<<v[i]<<" ";
  }
  cout<<"\n";*/
  cout<<"Array has been initialized! Merge sort started with OpenMP\n";
  clock_t begin = clock();
  v = mergesort(v, 1);
  cout<<"Finished in "<<((clock()-begin)*1000)/CLOCKS_PER_SEC<<"ms \n";
  /*for(int i = 0;i<size;++i){
      cout<<v[i]<<" ";
  }
  cout<<"\n";*/
}