1 #include <iostream>
2 #include <cstring>
3
4 using namespace std;
5 const int maxn = 50005;
6 int a[maxn];
7 int res[maxn];
8 int ans;
9
10 //归并排序
11 void merge(int l, int r){
12 //cout<<l<<" "<<r<<endl;
13 int mid = (l + r) >> 1;
14 int i = l, j = mid + 1;
15 int cur = l;
16 while (i <= mid && j <= r){
17 if (a[i] <= a[j])
18 res[cur++] = a[i++];
19 else{
20 res[cur++] = a[j++];
21 ans += mid - i + 1; //找到逆序的个数
22 }
23 }
24 while (i <= mid) res[cur++] = a[i++];
25 while (j <= r) res[cur++] = a[j++];
26 //排序好后
27 for (int i = l; i <= r; i++) a[i] = res[i];
28 }
29 void mer_sort(int l, int r){
30 if (l < r){
31 int mid = (l + r) >> 1;
32 mer_sort(l, mid); //分解
33 mer_sort(mid + 1, r); //分解
34 merge(l, r); //合并
35 }
36 }
37 int main()
38 {
39 int n;
40 while (cin >> n){
41 for (int i = 1; i <= n; i++) cin >> a[i];
42
43 ans = 0;
44 mer_sort(1, n);
45 cout << ans << endl;
46 }
47 return 0;
48 }
