逆序对代码

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int maxn=3e5;
int n,a[maxn+5],b[maxn+5],cnt[maxn+5],tmp[maxn+5],temp1[maxn+5],temp2[maxn+5];
ll ans;
void msort(int l,int r){
	if(l==r){
		return;
	}
	int mid=(l+r)/2;
	msort(l,mid);
	msort(mid+1,r);
	for(int i=l;i<=r;i++){
		cnt[b[i]]=0;
	}
	for(int i=l;i<=mid;i++){
		cnt[b[i]]++;
	}
	int i=l,j=mid+1,k=l;
	while(i<=mid&&j<=r){
		if(a[i]<=a[j]){
			cnt[b[i]]--;
			tmp[k++]=i++;
		}
		else{
			ans+=mid-i+1-cnt[b[j]];
			tmp[k++]=j++;
		}
	}
	while(i<=mid){
		tmp[k++]=i++;
	}
	while(j<=r){
		tmp[k++]=j++;		
	}
	for(int i=l;i<=r;i++){
		temp1[i]=b[i],temp2[i]=a[i];
	}
	for(int i=l;i<=r;i++){
		a[i]=temp2[tmp[i]],b[i]=temp1[tmp[i]];
	}
}
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>b[i];
	}
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	msort(1,n);
	cout<<ans;
	return 0;
}