[数据结构与算法]归并排序 - AcWing 788. 逆序对的数量（C++）- 简单

题目链接：https://www.acwing.com/problem/content/description/790/
题目如下：

#include <iostream>
#include <vector>
using namespace std;
using LL=long long;
const int N=100010;
LL arr[N];
LL merge(LL arr[],int l,int r);
int main(){
    int n;
    cin>>n;
    for(int i=0;i<n;i++) cin>>arr[i];
    
    cout<<merge(arr,0,n-1)<<endl;
    return 0;
}
LL merge(LL arr[],int l,int r){
    if(l>=r) return 0;
    int mid=(l+r)>>1;
    LL res=merge(arr,l,mid)+merge(arr,mid+1,r);
    int i=l,j=mid+1;
    vector<int> temp;
    
    while(i<=mid&&j<=r){
        if(arr[i]<=arr[j]) temp.push_back(arr[i++]);
        else {
            temp.push_back(arr[j++]);
            res+=mid-i+1;
        }
    }
    
    while(i<=mid) temp.push_back(arr[i++]);
    while(j<=r) temp.push_back(arr[j++]);
    
    i=l;
    for(auto e:temp) arr[i++]=e;
    
    return res;
}