hdu 1394 Minimum Inversion Number 【线段树求逆序数】

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<iostream>
 4 #include<algorithm>
 5 #include<cmath>
 6 #include<vector>
 7 using namespace std;
 8 #define lp (p << 1)
 9 #define rp (p << 1 | 1)
10 #define getmid(l,r) (l + (r - l) / 2)
11 
12 const int maxn = 50005;
13 struct node{
14     int l,r,s;
15 }t[4*maxn];
16 
17 int a[maxn],n;
18 
19 int calc(int a,int b) { return a+b; }
20 
21 void Push_up(int p){
22     t[p].s = calc(t[lp].s,t[rp].s);
23 }
24 
25 void Build_tree(int p,int l,int r){
26     t[p].l = l;
27     t[p].r = r;
28     if(l == r){ t[p].s = 0;return;}
29     int mid = getmid(l,r);
30     Build_tree(lp,l,mid);
31     Build_tree(rp,mid+1,r);
32     Push_up(p);
33 }
34 
35 void update(int p,int s,int w){//给s点加上w 
36     if(t[p].l == t[p].r) {
37         t[p].s += w;
38         return;
39     }
40     int mid = getmid(t[p].l,t[p].r);
41     if(s <= mid) update(lp,s,w);
42     else update(rp,s,w);
43     Push_up(p);        
44 }
45 
46 int sum(int p,int l,int r){
47     if(t[p].l == l && t[p].r == r) return t[p].s;
48     
49         int mid = getmid(t[p].l,t[p].r);
50         if(r <= mid) return sum(lp,l,r);
51         else if(mid < l) return sum(rp,l,r);
52          else return calc(sum(lp,l,mid),sum(rp,mid+1,r));
53 }
54 
55 int main(){
56     int T;
57     while(scanf("%d",&n) != EOF){
58         for(int i = 1;i <= n;i++) scanf("%d",&a[i]),a[i];
59         Build_tree(1,1,n);
60         
61         int ans = 0;
62         for(int i = 1;i <= n;i++){       
63             ans += sum(1,a[i]+1,n);update(1,a[i],1);
64       //      printf("ans = %d\n",ans);
65         }
66         
67         int res = ans;
68         for(int i = 1;i <= n;i++){
69             ans += n-2*a[i] - 1;
70             res = min(res,ans);
71         //    printf("res = %d\n",res);
72         }
73         printf("%d\n",res);
74     }
75     return 0;
76 }