这个是和老师之外的另一种做法，便于理解原理

#include <iostream>
using namespace std;

typedef long long LL;
const int N = 100010;
int n, 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 i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r)
    {
        if (q[i] <= q[j]) 
        {
        tmp[k ++] = q[i ++];
        res += j - mid - 1;
        }
        else 
        {
            tmp[k ++] = q[j ++];

        }
    }
    while (i <= mid) 
    {
        tmp[k ++] = q[i ++];
        res += r - mid;
    }
    while (j <= r) tmp[k ++] = q[j ++];
    for (int i = l, k = 0; i <= r; i ++, k ++)
    {
        q[i] = tmp[k];
    }
    return res;
}

int main()
{
    cin >> n;
    for (int i = 0; i < n; i ++)
    {
        scanf("%d", &q[i]);
    }
    cout << merge_sort(0, n - 1) << endl;
    return 0;
}