求逆序数 noj117

#include<iostream>
#include<cstdio>
using namespace std;
#define N 1000000010
#define M 1000010
__int64 sum;
//long long sum;
int a[M],L[M/2+1],R[M/2+1];

void merger(int beg,int mid,int end)
{
//	cout<<"A:"<<beg<<" "<<mid<<" "<<end<<endl;
	int L_len=mid-beg,R_len=end-mid-1;
	int i,j,k;
	for(i=0;i<=L_len;i++)
		L[i]=a[beg+i];
	for(j=0;j<=R_len;j++)
		R[j]=a[mid+j+1];
	L[L_len+1]=R[R_len+1]=N;
	i=j=0;
	for(k=beg;k<=end;k++)
	{
		if(L[i]<=R[j])
			a[k]=L[i++];
		else
		{
			a[k]=R[j++];
			sum+=L_len-i+1;
		}
	}
//	for(i=0;i<=end;i++)
//		cout<<a[i]<<" ";
//	cout<<endl<<endl;
}

void mergerSort(int beg,int end)
{
	int mid=(beg+end)>>1;
	if(beg<end)
	{
		mergerSort(beg,mid);
		mergerSort(mid+1,end);
		merger(beg,mid,end);
	}
}

int main()
{
//	freopen("in.txt","r",stdin);
	int t,n;
	cin>>t;
	while(t--)
	{
		sum=0;
		scanf("%d",&n);
		for(int i=0;i<n;i++)
		{
			scanf("%d",&a[i]);
		}
		mergerSort(0,n-1);
		printf("%Id64\n",sum);
	//	printf("%lld\n",sum);
	}
	return 0;
}