逆序對(模板)

逆序對板子，我們在求逆序的時候，僅僅需要進行在歸併排序上加上 mid - i + 1即可

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