poj2299--归并排序求解逆序对

//归并排序求解逆序对 
#include<iostream>
#include<stdio.h> 
#include<math.h>
#include<algorithm>
#include<stack>
#include<queue>
#include<string.h>
typedef long long int ll; 
using namespace std;

ll  n,sum;
ll  a[500005],temp[500005];

void Merge(ll a[],ll l,ll mid,ll r){
   ll i=l,j=mid+1,k=0;
   while(i<=mid&&j<=r){
      if(a[i]<a[j]){
        temp[k++]=a[i++];
      }
      else{
        temp[k++]=a[j++];
        sum+=mid-i+1;//求逆序对
      }
   }
   while(i<=mid)  temp[k++]=a[i++];
   while(j<=r)    temp[k++]=a[j++];

   for(ll i=l,k=0;i<=r;k++,i++)
    a[i]=temp[k];

}

void MergeSort(ll a[],ll l,ll r){
   ll mid;
   if(l<r){
       mid=(l+r)/2;
       MergeSort(a,l,mid);
       MergeSort(a,mid+1,r);
       Merge(a,l,mid,r);
   }
}
int main()
{
    while(~scanf("%lld",&n))
    {
        sum = 0;
        if(n == 0)
            break;
        for(int i = 0; i < n; i++)
            scanf("%lld",&a[i]);
        MergeSort(a,0,n-1);
        printf("%lld\n",sum);
    }
    
    
    
    return 0;
}