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 = 50005;
struct node{
    int l,r,s;
}t[4*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 = 0;return;}
    int mid = getmid(l,r);
    Build_tree(lp,l,mid);
    Build_tree(rp,mid+1,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+1,r));
}

int main(){
    int T;
    while(scanf("%d",&n) != EOF){
        for(int i = 1;i <= n;i++) scanf("%d",&a[i]),a[i];
        Build_tree(1,1,n);
        
        int ans = 0;
        for(int i = 1;i <= n;i++){       
            ans += sum(1,a[i]+1,n);update(1,a[i],1);
      //      printf("ans = %d
",ans);
        }
        
        int res = ans;
        for(int i = 1;i <= n;i++){
            ans += n-2*a[i] - 1;
            res = min(res,ans);
        //    printf("res = %d
",res);
        }
        printf("%d
",res);
    }
    return 0;
}