直接算逆序对即可 改都不用改

#include <iostream>
#include <cstring>
#include <algorithm>

typedef long long LL;

using namespace std;

const int N = 1e6;
int q[N], tmp[N];
int n;

LL m_sort(int q[], int l, int r) {
    if (l >= r) return 0;

    int mid = l + r >> 1;
    LL res = m_sort(q, l, mid) + m_sort(q, 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 (i = l, j = 0; i <= r; i ++, j ++) q[i] = tmp[j];

    return res;
}

int main()
{
    cin >> n;
    for (int i = 0; i < n; i ++) cin >> q[i];

    cout << m_sort(q, 0, n - 1);

    return 0;
}