P80595: How many inversions?

#include <iostream>
#include <vector>
using namespace std;


int merge(vector<int>& v, int l, int m, int r)
//Return inversions and merged vector v[l..r] = v[l..m] ~ v[m+1..r]
{
    vector<int> z(r-l+1);
    int i = l;
    int j = m+1;
    int k = 0;
    int count = 0;
    while (i <= m && j <= r) {
        if (v[i] <= v[j])
            z[k++] = v[i++];
        else {
            z[k++] = v[j++];
            count += m+1 - i;
        }
    }
    while (i <= m) z[k++] = v[i++];
    while (j <= r) z[k++] = v[j++];
    for (k = 0; k < z.size(); ++k) v[l+k] = z[k];
    return count;
}

int mergesort_inv(vector<int>& v, int l, int r)
//Return number of inversions of v[l..r]
//Total ammount of inversions will be inversions of left vector + right vector (recursivity) + merge process inversions
{
    int count = 0;
    if (l < r) {
        int m = (l+r)/2;
        count = mergesort_inv(v, l, m);
        count += mergesort_inv(v, m+1, r);
        count += merge(v, l, m, r);
    }
    return count;
}

int main()
{
    int n;
    while (cin >> n) {
        vector<int> v(n);
        for (int i = 0; i < n; ++i) cin >> v[i];
        cout << mergesort_inv(v, 0, v.size()-1) << endl;
    }
}

//JosepRivaille