时间复杂度 $O(n)$

#include <iostream>

using namespace std;

typedef long long LL;

const int N = 1e5 + 10;
int n;
int q[N], tmp[N];

LL merge_sort(int l, int r) { //返回值是LL
    if (l >= r) return 0; //归并排序模板 l>=r可、l==r可，  快排模板 l>=r可、l==r不可
    int mid = l + r >> 1;
    int k = 0; //归并排序
    LL res = merge_sort(l, mid) + merge_sort(mid + 1, r);
    int i = l, j = mid + 1;
    while (i <= mid && j <= r) {
        if (q[i] <= q[j]) tmp[k ++] = q[i ++]; //这里是<=,容易写成<
        else {
            tmp[k ++] = q[j ++];
            res += mid - i + 1; //sj = mid - i + 1; 这里容易忘记是 +=！！！
        }
    }
    //扫尾
    while (i <= mid) tmp[k ++] = q[i ++];
    while (j <= r) tmp[k ++] = q[j ++];
    //物归原主
    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;
}