《逆序对的数量》

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 100010;
int q[N], tmp[N];
ll merge_sort(int l , int r)
{
    if (l >= r) return 0;
    int mid = l + r >> 1;
    ll res = merge_sort(l, mid) + merge_sort(mid + 1, r);
    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r)
    {
        if (q[i] <= q[j]) tmp[k ++] = q[i ++];
        else
        {
            tmp[k ++] = q[j ++];
            res += mid - i + 1;
        }
    }
    while (i <= mid) tmp[k ++] = q[i ++];
    while (j <= r) tmp[k ++] = q[j ++];
    for (int i = l, j = 0; i <= r; i ++, j ++)
    q[i] = tmp[j];
    return res;
}
void solve()
{
    int n;
    cin >> n;
    for (int i = 0; i < n; i ++) cin >> q[i];
    cout << merge_sort(0, n - 1) << endl;
    memset(q, 0, sizeof(q));
}
int main()
{
    int t;
    cin >> t;
    while (t --)
    {
        solve();
    }
    return 0;
}