poj 2299 Ultra-QuickSort(归并排序求逆序数)

#include<iostream>
#include<cstdio>
using namespace std;
int n;
__int64 ans;
int arr[1001000],arr1[1001000];
void Merge(int s,int m,int t);
void MergeSort(int s,int t)
{
if(s==t)
{
arr1[s]=arr[s];
}
else
{
int m=(s+t)/2;
MergeSort(s,m);
MergeSort(m+1,t);
Merge(s,m,t);
}
}
void Merge(int s,int m,int t)
{
int i=s,j=m+1,k=s;
while(i<=m&&j<=t)
{
if(arr[i]<=arr[j])
{
arr1[k++]=arr[i++];
}
else
{
arr1[k++]=arr[j++];
ans+=m-i+1;//注意这，，，arr[i]>arr[j],则从arr[i]到arr[m]都是逆序数
}
}
if(i<=m)
while(i<=m)
{
arr1[k++]=arr[i++];
//ans++;
}
else
while(j<=t)
arr1[k++]=arr[j++];
for(int p=s;p<=t;p++)
arr[p]=arr1[p];
}
int main()
{
while(scanf("%d",&n)!=EOF&&n!=0)
{
for(int i=1;i<=n;i++)
scanf("%d",&arr[i]);
ans=0;
MergeSort(1,n);

/*for(int i=1;i<=n;i++)
printf("%d   ",arr[i]);
printf("\n");*/

printf("%I64d\n",ans);
}
//system("pause");
return 0;
}