Binary Tree — Invert a Tree

Mirror a binary tree: at every node, swap the left and right children.
solve(levelOrder) builds the tree from a level-order array, inverts it and
returns it serialized again with the provided toLevelOrder helper.

Examples

solve([4, 2, 7, 1, 3, 6, 9])  → [4, 7, 2, 9, 6, 3, 1]
solve([2, 1, 3])              → [2, 3, 1]
solve([1, 2])                 → [1, null, 2]
solve([])                     → []

Swapping only the root's children is the classic mistake — the swap has to
happen all the way down.