HDU 1394 Minimum Inversion Number【 树状数组 】

 1 #include<iostream>  
 2 #include<cstdio>  
 3 #include<cstring> 
 4 #include <cmath> 
 5 #include<stack>
 6 #include<vector>
 7 #include<map> 
 8 #include<set>
 9 #include<queue> 
10 #include<algorithm>  
11 using namespace std;
12 
13 typedef long long LL;
14 const int INF = (1<<30)-1;
15 const int mod=1000000007;
16 const int maxn=50005;
17 
18 int n;
19 int a[maxn],c[maxn];
20 
21 int lowbit(int x){ return x &(-x);}
22 
23 int sum(int x){
24     int ret =0;
25     while(x>0){
26         ret+=c[x];x-=lowbit(x);
27     }
28     return ret;
29 }
30 
31 void add(int x,int d){
32     while(x<=n){
33         c[x]+=d;x+=lowbit(x);
34     }
35 }
36 
37 int main(){
38     while(scanf("%d",&n) != EOF){
39         memset(c,0,sizeof(c));
40         for(int i=1;i<=n;i++) scanf("%d",&a[i]),a[i]++;
41         
42         int ans=0;
43         for(int i=1;i<=n;i++){
44             ans += i - 1-sum(a[i]);
45         //    printf("ans=%d\n",ans);
46             add(a[i],1);
47         }
48         int minn=INF;
49         for(int i=1;i<=n;i++){
50             ans += n-2*a[i] + 1;
51             minn=min(minn,ans);
52         }
53         printf("%d\n",minn);
54     }
55     return 0;
56 }