归并排序+逆序数（模板）

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 const int MAXN=1e5+10;
 4 int a[MAXN];
 5 int temp[MAXN];
 6 long long  ans;
 7 void merge(int l,int mid,int r)
 8 {
 9     int i=l;
10     int j=mid+1;
11     int k=l;
12     while(i<=mid&&j<=r)
13     {
14         if(a[i]<=a[j])
15         {
16             temp[k++]=a[i++];
17         }
18         else
19         {
20             ans+=j-k;//记录数量;
21             temp[k++]=a[j++];
22         }
23     }
24     while(i<=mid)
25         temp[k++]=a[i++];
26     while(j<=r)
27         temp[k++]=a[j++];
28     for (int s = l; s <=r; ++s) {
29         a[s]=temp[s];
30     }
31 
32 }
33 
34 void mergesort(int l,int r)
35 {
36     if(l<r)
37     {
38         int mid=(l+r)/2;
39         mergesort(l,mid);
40         mergesort(mid+1,r);
41         merge(l,mid,r);
42     }
43 }
44 int main()
45 {
46     int n,x,y;
47     while(scanf("%d%d%d",&n,&x,&y)!=EOF) {
48         ans=0;
49         memset(temp,0,sizeof(temp));
50         for (int i = 0; i <n ; ++i) {
51             scanf("%d",&a[i]);
52         }
53         mergesort(0,n-1);
54         printf("%lld
",ans);
55     }
56     return 0;
57 }