HDU 1394 Minimum Inversion Number

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 const int maxn = 5005;
 4 
 5 int d[maxn],tree[maxn<<2],n;
 6 void update(int L,int R,int id,int v) {
 7     if(id <= L && id >= R) {
 8         tree[v]++;
 9         return;
10     }
11     int mid = (L + R)>>1;
12     if(id <= mid) update(L,mid,id,v<<1);
13     if(id > mid) update(mid+1,R,id,v<<1|1);
14     tree[v] = tree[v<<1] + tree[v<<1|1];
15 }
16 int query(int L,int R,int lt,int rt,int v){
17     if(lt <= L && rt >= R) return tree[v];
18     int mid = (L + R)>>1,ans = 0;
19     if(lt <= mid) ans += query(L,mid,lt,rt,v<<1);
20     if(rt > mid) ans += query(mid+1,R,lt,rt,v<<1|1);
21     return ans;
22 }
23 int main() {
24     while(~scanf("%d",&n)) {
25         memset(tree,0,sizeof tree);
26         int ans = 0,tmp = 0;
27         for(int i = 0; i < n; ++i) {
28             scanf("%d",d+i);
29             tmp += query(0,n-1,d[i],n-1,1);
30             update(0,n-1,d[i],1);
31         }
32         ans = tmp;
33         for(int i = 0; i < n; ++i){
34             tmp += n - 1 - d[i] - d[i];
35             ans = min(ans,tmp);
36         }
37         printf("%d\n",ans);
38     }
39     return 0;
40 }