CountInv w/ MergeSort

#include <iostream>
#include <queue>
#include <vector>
using namespace std;
using ull = unsigned long long;

ull ans;
vector <ull> arr;

ull merge (ull b, ull m, ull e) {
    queue <ull> left;
    queue <ull> right;
    for (ull i = b; i <= m; ++i) left.push(arr[i]);
    for (ull i = m + 1; i <= e; ++i) right.push(arr[i]);
    ull idx = b, out = 0;
    while (!left.empty() && !right.empty()) {
        if (left.front() <= right.front()) {
            arr[idx++] = left.front();
            left.pop();
        }
        else {
            arr[idx++] = right.front();
            right.pop();
            out += left.size();
        }
    }
    while (!left.empty()) {
        arr[idx++] = left.front();
        left.pop();
    }
    while (!right.empty()) {
        arr[idx++] = right.front();
        right.pop();
    }
    return out;
}

void mergesort(ull b, ull e) {
    if (b < e) {
        ull m = ((b + e) / 2);
        mergesort(b, m);
        mergesort(m + 1, e);
        ans += merge(b, m, e);
    }
}

int main() {
    ull n; cin >> n;
    for (ull i = 0; i < n; ++i) {
        ull e; cin >> e;
        arr.push_back(e);
    }
    mergesort(0, n - 1);
    cout << ans;
}