30分求助

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<ctime>
#include<cstdlib>
#include<queue>
#include<vector>
#define ll long long
#define INF 2147483647
using namespace std;
int n,a[400100],t[400100],ans;
void merge_sort(int l,int r)
{
	int mid=(l+r)/2;
	if(l==r)return;
	merge_sort(l,mid);
	merge_sort(mid+1,r);
	for(int i=l,j=l,k=mid+1;i<=r;i++)
	{
		if(j==mid+1)t[i]=a[k++];
		else if(k==r+1)t[i]=a[j++];
		else if(a[j]<a[k])t[i]=a[j++];
		else t[i]=a[k++],ans+=mid-j+1;;
	}
	for(int i=l;i<=r;i++)a[i]=t[i];
}
int main()
{
	cin>>n;
	for(int i=n;i>=1;i--)cin>>a[i];
	merge_sort(1,n);
	cout<<ans<<endl;
	return 0;
}