第1行：N，N為序列的長度（n <= 50000)
第2 - N + 1行：序列中的元素（0 <= A[i] <= 10^9）

輸出逆序數

4
2
4
3
1

4

看程序吧  不會講

 1 #include <iostream>
 2 using namespace
 
 std;
 3 #include<string.h>
 4 #include<set>
 5 #include<stdio.h>
 6 #include<math.h>
 7 #include<queue>
 8 #include<map>
 9 #include<algorithm>
10 #include<cstdio>
11 #include<cmath>
12 #include<cstring>
13 #include <cstdio>
14 #include <cstdlib>
15
 
 #include<stack>
16 #include<vector>
17 int a[51000];
18 int b[51000];
19 long long sum=0;
20 void dfs1(int kaishi,int zhongjian,int jieshu)
21 {
22     int i=kaishi,j=zhongjian+1;
23     int k=kaishi;
24     while(i<=zhongjian&&j<=jieshu)
25     {
26         if(a[i]<a[j])
27             b[k++]=a[i++];
 
28         else
29         {
30 
31             b[k++]=a[j++];
32             sum+=zhongjian-i+1;
33             //cout<<j<<"_"<<k<<endl;
34             continue;
35         }
36     }
37     while(i<=zhongjian)
38     {
39         b[k++]=a[i++];
40     }
41     while(j<=jieshu)
42     {
43         b[k++]=a[j++];
44     }
45     for(i=kaishi;i<=jieshu;i++)
46         a[i]=b[i];
47 }
48 void dfs(int kaishi,int jieshu)
49 {
50     if(kaishi<jieshu)
51     {
52         int zhongjian=(kaishi+jieshu)/2;
53         dfs(kaishi,zhongjian);
54         dfs(zhongjian+1,jieshu);
55         dfs1(kaishi,zhongjian,jieshu);
56     }
57 }
58 int main()
59 {
60     int n;
61     scanf("%d",&n);
62     for(int i=0;i<n;i++)
63         {
64             scanf("%d",&a[i]);
65             b[i]=a[i];
66         }
67     dfs(0,n-1);
68     cout<<sum<<endl;
69     return 0;
70 }

逆序數 51nod 1019 歸並 分治