/c/
/c/aa/
/c/bb/
/c/cc/
/c/a1.mp3      <<<< file! it should at the end of the list
/c/a2.mp3      <<<< file! should at the end of the list
/c/bb/aa1.mp3  <<<< this should be below /c/bb/
/c/bb/aa2.mp3  <<<< this should be below /c/bb/