1 #include <iostream>
2 const int MAX = 500000;
3 int a[MAX];
4 int swap[MAX]; //临时数组
5 int n; //数组a的长度
6 __int64 result; //数组a中的逆序数
7
8 //归并两个已经有序的段：a[low]—a[mid]和a[mid+1]—a[high]，使得a[low]—a[high]有序。
9 void merge(int low, int mid, int high)
10 {
11 int i = low;
12 int j = mid + 1;
13 int m = 0;
14 while(i <= mid && j <= high)
15 {
16 if(a[i] <= a[j])
17 {
18 swap[m++] = a[i];
19 i++;
20 }
21 else
22 {
23 swap[m++] = a[j];
24 j++;
25 result += mid - i + 1;
26 }
27 }
28 while(i <= mid)
29 {
30 swap[m++] = a[i];
31 i++;
32 }
33 while(j <= mid)
34 {
35 swap[m++] = a[j];
36 j++;
37 }
38 for(i = 0; i < m; i++)
39 a[low + i] = swap[i];
40 }
41
42 //归并排序：对a[low]—a[high]进行归并排序。
43 void mergeSort(int low, int high)
44 {
45 int mid;
46 if(low < high)
47 {
48 mid = (low + high) /2;
49 mergeSort(low, mid);
50 mergeSort(mid + 1, high);
51 merge(low, mid, high);
52 }
53 }
54
55 int main()
56 {
57 int i;
58 while(true)
59 {
60 scanf("%d",&n);
61 if(n == 0) break;
62 result = 0;
63 for(i = 0; i < n; i++)
64 scanf("%d",a+i);
65 mergeSort(0, n-1);
66 printf("%I64d\n",result);
67 }
68 return 0;
69 }
