Divide and Conquer count inversions of an array

CountInversionsOfArray.cpp

// Explanation https://www.youtube.com/watch?v=owZhw-A0yWE

#include <vector>
#include <iostream>

using namespace std;
int inv_count = 0; // NAIV, since I didn't want to modify my inital merge sort impelemntation LOL

vector<int> merge(vector<int> left, vector<int> right) {
    vector<int> sortedArray(left.size() + right.size(), 0);
    int i = 0; /* i is index for left subarray*/
    int j = 0; /* j is index for right subarray*/
    int k = 0; /* k is index for resultant merged subarray*/
    while (i < left.size() && j < right.size()) {
        if (left[i] <= right[j]) {
            sortedArray[k++] = left[i++];
        } else {
            sortedArray[k++] = right[j++];
            // This is were the magic happens, left.size() - i since you have to include all the possibilites.
            inv_count = inv_count + ((int)left.size() - i);
        }
    }
    cout << inv_count << endl;
    while (i < left.size()) sortedArray[k++] = left[i++];
    while (j < right.size()) sortedArray[k++] = right[j++];
    return sortedArray;
}

vector<int> mergeSort(vector<int> array) {
    if (array.size() <= 1) return array;
    int n = (int)array.size();
    int mid = n / 2;
    vector<int> left(array.begin(), array.begin() + mid);
    vector<int> right(array.begin() + mid, array.end());
    return merge(mergeSort(left), mergeSort(right));
}

int countInversions(vector<int> arr) {
    return  0;
}

int main() {
    vector<int> arr = { 1, 20, 6, 4, 5 };
    mergeSort(arr);
    cout << "number of inversions: " << inv_count << endl;
    return 0;
}