[]
[NULL]
[NULL,1]
[NULL,1,NULL]
[NULL,1,NULL,3]
[NULL,1,NULL,3,NULL]
[NULL,1,NULL,3,NULL,5]
[NULL,1,NULL,3,NULL,5,NULL]
[NULL,1,NULL,3,NULL,5,NULL,7]
[NULL,1,NULL,3,NULL,5,NULL,7,NULL]