逆序对的数量（归并排序模板y神）

#include<iostream>
using namespace std;
typedef long long LL;
const int N = 100010;
int n;
int q[N],tmp[N];
LL merge_sort(int l, int r)
{
	if(l>=r)return 0;
	int mid = (l+r)/2;//l+r >> 1;等价
	LL res = merge_sort(l,mid)+merge_sort(mid+1,r);
	//归并
	int k = 0,i = 1,j = mid+1;
	while(i<=mid&&j<=r){
		if(q[i]<=q[j])temp[k++]=q[i++];
		else{
			tmp[k++]=tmp[j++};
			res = 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(int 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>>q[i];
	cout<<merge_sort(0,n-1)<<endl;
	return 0;
}