Сортировка слиянием

#include <iostream>
#include <vector>

using namespace std;

vector<int> merge(const vector<int> &a, const vector<int> &b)
{
    int a_size = a.size();
    int b_size = b.size();
    vector<int> c(a_size + b_size);
    int i = 0, j = 0, k = 0;
    while(i<a_size||j<b_size)
    {
        if (j == b_size || (i < a_size && a[i] < b[j]))
            c[k++] = a[i++];
        else
            c[k++] = b[j++];
    }
    return c;
}

vector<int> mergeSort(const vector<int> &a, int l, int r)
{
    int n = r-l;
    if (n <= 1)
        return vector<int> (a.begin()+l,a.begin()+r);
    int mid = (l + r) / 2;
    vector<int> a_l = mergeSort(a, l, mid);
    vector<int> a_r = mergeSort(a, mid, r);
    return merge(a_l, a_r);
}
int main()
{
    int number_1;

    cin >> number_1;
    vector<int> array_1(number_1);
    for (auto& x : array_1)
    {
        cin >> x;
    }


    array_1 = mergeSort(array_1, 0, number_1);

    for (auto& x : array_1)
    {
        cout << x << " ";
    }

    return 0;
}