- []

[[]]

- [1]

[[1]]

- [1,2]

[[1,2] , [2,1]]


- [1,2,3]

[[1,2,3], [1,3,2], [2,1,3], [2,3, 1], [3,1,2], [3,2,1]]

- [1,2,3,4]

[[1,2,3,4], [1,2,4,3], ...]