Merge sort and inversion counting

#include <bits/stdc++.h>

using namespace std;

vector<int>v;
int n;

long long int count_inversion(int beg,int en)
{
    if(beg>=en)
        return 0;
    int md;
    md=(beg+en)/2;
    long long int inversion_left=count_inversion(beg,md);
    long long int inversion_right=count_inversion(md+1,en);
    vector<int>tmp(en-beg+1);
    long long int inversion_merge=0;
    int i=beg,j=md+1,k=0;
    while(i<=md&&j<=en)
    {
        if(v[i]<=v[j])
        {
            tmp[k]=v[i];
            i++;
        }
        else
        {
            inversion_merge=inversion_merge+(md-i+1);
            tmp[k]=v[j];
            j++;
        }
        k++;
    }
    while(i<=md)
    {
        tmp[k]=v[i];
        i++;
        k++;
    }
    while(j<=en)
    {
        tmp[k]=v[j];
        j++;
        k++;
    }
    i=0;
    while(beg<=en)
    {
        v[beg]=tmp[i];
        i++;
        beg++;
    }
    tmp.clear();
    return inversion_left+inversion_right+inversion_merge;
}


int main()
{
    int i,j;
    cin>>n;
    for(i=0;i<n;i++)
    {
        cin>>j;
        v.push_back(j);
    }
    cout<<count_inversion(0,n-1)<<endl;
    return 0;
}