1 #include<bits/stdc++.h>
2 using namespace std;
3 int n;
4 long long a[500005],b[500005],ans;//a为待排序数组,b为临时数组,ans为逆序对数
5 void mergesort(int l,int r)//l为左边界,r为右边界
6 {
7 if(l==r) return;
8 int mid=(l+r)>>1;
9 mergesort(l,mid);
10 mergesort(mid+1,r);
11 int i=l,j=mid+1,k=l;
12 while(i<=mid&&j<=r)
13 {
14 if(a[i]<=a[j]) b[k++]=a[i++]; //注意等号 若a[i]==a[j] 则不构成逆序对
15 else b[k++]=a[j++],ans+=mid-i+1;//求逆序对
16 }
17 while(i<=mid) b[k++]=a[i++];
18 while(j<=r) b[k++]=a[j++];
19 for(int i=l;i<=r;i++) a[i]=b[i];
20 }
21 int main()
22 {
23 scanf("%d",&n);
24 while(n)
25 {
26 ans=0;
27 memset(a,0,sizeof(a));
28 memset(b,0,sizeof(b));
29 for(int i=1;i<=n;i++) scanf("%d",&a[i]);
30 mergesort(1,n);
31 printf("%lld\n",ans);
32 scanf("%d",&n);
33 }
34 return 0;
35 }
