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

之前写过树状数组的，再用线段树写一下～～～

 #include<cstdio>
 #include<cstring>
 #include<iostream>
 #include<algorithm>
 #include<cmath>
 #include<vector>
 using namespace std;
 #define lp (p << 1)
 #define rp (p << 1 | 1)
 #define getmid(l,r) (l + (r - l) / 2)

 const int maxn = ;
 struct node{
     int l,r,s;
 }t[*maxn];

 int a[maxn],n;

 int calc(int a,int b) { return a+b; }

 void Push_up(int p){
     t[p].s = calc(t[lp].s,t[rp].s);
 }

 void Build_tree(int p,int l,int r){
     t[p].l = l;
     t[p].r = r;
     if(l == r){ t[p].s = ;return;}
     int mid = getmid(l,r);
     Build_tree(lp,l,mid);
     Build_tree(rp,mid+,r);
     Push_up(p);
 }

 void update(int p,int s,int w){//给s点加上w
     if(t[p].l == t[p].r) {
         t[p].s += w;
         return;
     }
     int mid = getmid(t[p].l,t[p].r);
     if(s <= mid) update(lp,s,w);
     else update(rp,s,w);
     Push_up(p);
 }

 int sum(int p,int l,int r){
     if(t[p].l == l && t[p].r == r) return t[p].s;

         int mid = getmid(t[p].l,t[p].r);
         if(r <= mid) return sum(lp,l,r);
         else if(mid < l) return sum(rp,l,r);
          else return calc(sum(lp,l,mid),sum(rp,mid+,r));
 }

 int main(){
     int T;
     while(scanf("%d",&n) != EOF){
         for(int i = ;i <= n;i++) scanf("%d",&a[i]),a[i];
         Build_tree(,,n);

         int ans = ;
         for(int i = ;i <= n;i++){
             ans += sum(,a[i]+,n);update(,a[i],);
       //      printf("ans = %d\n",ans);
         }

         int res = ans;
         for(int i = ;i <= n;i++){
             ans += n-*a[i] - ;
             res = min(res,ans);
         //    printf("res = %d\n",res);
         }
         printf("%d\n",res);
     }
     return ;
 }