hdu 1394 Minimum Inversion Number（线段树or树状数组）

const int maxn = 50005;
int sum[maxn<<2];

int n;


void PushUp(int rt){
    sum[rt] = sum[rt<<1] + sum[rt<<1|1];
}

void build(int l,int r,int rt){
    sum[rt] = 0;
    if(l==r) return;
    int m = (l + r) >> 1;
    build(lson);
    build(rson);
}

void update(int p,int l,int r,int rt){
    if(l==r){
        ++sum[rt];
        return;
    }
    int m = (l + r) >> 1;
    if(p <= m) update(p,lson);
    else update(p,rson);
    PushUp(rt);
}

int query(int L,int R,int l,int r,int rt){
    if(L<=l && r<=R){
        return sum[rt];
    }
    int m = (l + r) >> 1;
    int ret = 0;
    if(L <= m) ret += query(L,R,lson);
    if(R > m) ret += query(L,R,rson);
    return ret;
}



int main(){

    int a[maxn];

    while(scanf("%d",&n)!=EOF){

        build(0,n-1,1);
        int sum = 0;
        for(int i=1;i<=n;++i){
            scanf("%d",&a[i]);
            sum += query(a[i]+1,n-1,0,n-1,1);
            update(a[i],0,n-1,1);
        }

        //printf("%d\n",sum);


        int ans = sum;
        for(int i=1;i<n;++i){
            sum += ( n - a[i] -1 - a[i] );
            ans = min( ans,sum );
        }

        printf("%d\n",ans);

    }

}