$\huge \color{orange}{成仙之路->}$ $\huge \color{purple}{算法基础课题解}$

思路：

逆序对数量：res = mid - i + 1

完整代码

#include<bits/stdc++.h>

using namespace std;

typedef long long LL;

const int N = 100010;

int n;
int a[N],tmp[N];

LL merge_sort(int q[],int l,int r)
{
    if(l>=r) return 0;

    int mid=l+r>>1;
    LL res=merge_sort(q,l,mid)+merge_sort(q,mid+1,r);

    //归并过程
    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;
        }

    //扫尾
    while(i<=mid) tmp[k++]=q[i++];
    while(j<=r) tmp[k++]=q[j++];

    //物归原主
    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>>a[i];

    cout<<merge_sort(a,0,n-1)<<endl;

    return 0;
}