HDU1394 Minimum Inversion Number

 1 /*by SilverN*/
 2 #include<algorithm>
 3 #include<iostream>
 4 #include<cstring>
 5 #include<cstdio>
 6 #include<cmath>
 7 using namespace std;
 8 const int mxn=12000;
 9 int t[mxn];
10 int n;
11 int read(){
12     int x=0,f=1;char ch=getchar();
13     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
14     while(ch>=‘0‘ && ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}
15     return x;
16 }
17 int lowbit(int x){
18     return x&-x;
19 }
20 void add(int pos,int v){
21     while(pos<=n){
22         t[pos]+=v;
23         pos+=lowbit(pos);
24     }
25     return;
26 }
27 int ask(int x){
28     int res=0;
29     while(x){
30         res+=t[x];
31         x-=lowbit(x);
32     }
33     return res;
34 }
35 int ans=1e8;
36 int a[mxn];
37 int main(){
38     while(scanf("%d",&n)!=EOF){
39         memset(t,0,sizeof t);
40         ans=1e8;
41         int i,j;
42         int res=0;
43         for(i=1;i<=n;i++){
44             a[i]=read();
45             a[i]++;
46             res+=ask(n)-ask(a[i]);
47             add(a[i],1);
48         }
49         ans=min(ans,res);
50         for(i=1;i<=n;i++){
51             res+=(n-a[i])-(a[i]-1);
52             ans=min(ans,res);
53         }
54         printf("%d\n",ans);
55     }
56     return 0;
57 }