容易发现:交换次数 = 逆序对数量,用归并排序

#include<iostream>
using namespace std;
const int N = 100010;
int n;
int q[N],tmp[N];
int m_sort(int l,int r)
{
    if(l >= r) return 0;

    int mid = l + r >> 1;
    int res = 0;
    res = m_sort(l,mid) + m_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{
            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 , k = 0 ; i <= r ; i ++ , k ++) q[i] = tmp[k];
    return res;
}
int main()
{
    scanf("%d",&n);
    for(int i = 0 ; i < n ; i ++) scanf("%d",&q[i]);
    printf("%d",m_sort(0,n - 1));
    return 0;
}