choroba's solution looks good -- nay, elegant -- assuming the directories you intend to scan are actually in an alphanumeric order. But if your desired order is:

dir_
    suba
    subb
    ....

dir23
    sub1
    sub2

directoryA
    subdir0
    subdir1
    subdir11
[download]

or some other scheme where an asciibetic order would be UNsatisfactory, then you may need to customize your plan.