逆序对

void qiu(int l,int r)
{
    if(l>=r) return;
    int mid=(l+r)/2;
    qiu(l,mid);
    qiu(mid+1,r);
    int ll=l,rr=mid+1,k=1;
    while(ll<=mid&&rr<=r)
    {
        if(a[ll]>a[rr])
        {
            b[k]=a[rr];
            rr++;
            k++;
            ans+=mid-ll+1;
        }
        else
        {
            b[k]=a[ll];
            ll++;
            k++;
        }
    }
    while(ll<=mid)
    {
        b[k]=a[ll];
        ll++;
        k++;
    }
    while(rr<=r)
    {
        b[k]=a[rr];
        rr++;
        k++;
    }
    for(int i=l,j=1;i<=r;i++,j++)    a[i]=b[j];
}