merge_sort(l, r)返回区间[l, r]内逆序对的个数，而区间[l, r]内的逆序对个数是左半边的逆序对个数merge_sort(l, mid)和右半边逆序对个数merge_sort(mid + 1, r)之和外加左右两边构成的逆序对个数。

#include <iostream>
using namespace std;

const int N = 100010;

#define LL long long

int n;
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 i = l, j = mid + 1, k = 0;
    while(i <= mid && j <= r)
        if(q[i] <= q[j]) tmp[k ++] = q[i ++];
        else{
            res += mid - i + 1; // 1
            tmp[k ++] = q[j ++];
        }
    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 << merge_sort(0, n - 1);
    
    return 0;
}

注意：1处不能写成j - mid，否则会出现下面的情况

788. 逆序对的数量