题解

#include<iostream>
using namespace std;
int n,a[500010],temp[500010];
long long sum=0;
void MergeSort(int a[],int l,int r){
	if(l>=r){
        return ;
    } 
	int mid=(l+r)/2;
	MergeSort(a,l,mid);
	MergeSort(a,mid+1,r);
	int i=l,j=mid+1,k=l;
	while(i<=mid&&j<=r){
		if(a[i]<=a[j]){
			temp[k++]=a[i++];
        }else{
			temp[k++]=a[j++];
            sum+=mid-i+1;
        }
	}
	while(i<=mid){
		temp[k++]=a[i++];
    }
	while(j<=r){
		temp[k++]=a[j++];
    }
	for(int i=l;i<=r;i++){
        a[i]=temp[i];
    }
}
int main(){ 
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
    }
	MergeSort(a,1,n);
    cout<<sum;
	return 0;
}