51Nod 1019 逆序数 (归并排序)

 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 }