Problem D (inversions), contest 3

#include <bits/stdc++.h>
using namespace std;
int inv=0;
vector <long long> mergesort(vector <long long> v, int i, int j){
    if (i==j) {
        vector<long long> ans;
        ans.push_back(v[i]);
        return ans;
    }
    int mid = (i+j)/2;
    vector <long long> lv = mergesort(v, i, mid), rv = mergesort(v, mid+1, j), ans;
    int l=0, r=0;
    while (l<mid-i+1 or r<j-mid) {
        if(l>=mid-i+1) ans.push_back(rv[r++]);
        else if(r>=j-mid) ans.push_back(lv[l++]);
        else if(rv[r]>=lv[l]) ans.push_back(lv[l++]);
        else ans.push_back(rv[r++]), inv+=mid-i+1-l;
    }
    return ans;
}
int main(){
    ios_base::sync_with_stdio (0);
    cin.tie(0);
    int t, n, x;
    cin >> t;
    for(int i=1; i<=t; i++){
        cin >> n;
        inv=0;
        vector <long long> v;
        for(int j=1; j<=n; j++){
            cin >> x;
            v.push_back(x);
        }
        mergesort(v, 0, n-1);
        cout <<inv<<endl;
    }
}