树状数组求逆序数

 1 #include <stdio.h>
 2 #include <string.h>
 3 #include <iostream>
 4 #include <algorithm>
 5 #include <vector>
 6 #include <queue>
 7 #include <stack>
 8 #include <set>
 9 #include <map>
10 #include <string>
11 #include <math.h>
12 #include <stdlib.h>
13 using namespace std;
14 #define LL long long
15 struct node
16 {
17     LL v,id;
18 }p[500005];
19 LL f[500005],n;
20 LL d[500005];
21 bool cmp(node a,node b)
22 {
23     return a.v<b.v;
24 }
25 int lowbit(int x)
26 {
27     return x&(-x);
28 }
29 void update(int x)
30 {
31     while(x<=n)
32     {
33         f[x]++;
34         x+=lowbit(x);
35     }
36     return;
37 }
38 LL sum(int x)
39 {
40     LL s=0;
41     while(x)
42     {
43         s+=f[x];
44         x-=lowbit(x);
45     }
46     return s;
47 }
48 int main()
49 {
50     LL i,j,k,m;
51     while(scanf("%lld",&n)!=EOF)
52     {
53         if(n==0) break;
54         memset(f,0,sizeof(f));
55         memset(d,0,sizeof(d));
56         for(i=1;i<=n;i++)
57         {
58             scanf("%lld",&p[i].v);
59             p[i].id=i;
60         }
61         sort(p+1,p+n+1,cmp);
62         for(i=1;i<=n;i++)
63             d[p[i].id]=i;
64         LL ans=0;
65         for(i=1;i<=n;i++)
66         {
67             update(d[i]);
68             ans+=(i-sum(d[i]));
69         }
70         printf("%lld\n",ans);
71     }
72     return 0;
73 }