Mirror Tree

[
  [
    1,
    2,
    3,
    4,
    5
  ]
]

Explanation. The original tree structured as `[1, 2, 3, 4, 5]` where 1 is the root, 2 is its left child, 3 is its right child, 4 is the left child of 2, and 5 is the right child of 2. Mirroring it swaps all left and right children, resulting in `[1, 3, 2, 5, 4]`.

[
  [
    4,
    2,
    7,
    1,
    3,
    6,
    9
  ]
]

Explanation. Swapping every left and right child nodes transforms the tree `[4, 2, 7, 1, 3, 6, 9]` to `[4, 7, 2, 9, 6, 3, 1]`.

[
  [
    2,
    3,
    1
  ]
]

Explanation. Mirror image of tree `[2, 3, 1]` results in `[2, 1, 3]` where node 3 and 1 are swapped.

[
  [
    1
  ]
]

Explanation. Mirroring a single node tree results in the same tree `[1]`.

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

Explanation. Original tree is `[1, null, 2, null, 3]` with nodes increasing to the right. After mirroring, the nodes accumulate to the left resulting in `[1, 2, null, 3]`.