Binary Tree — Zigzag Level Order

Return the level-order traversal of a binary tree, but alternate the direction on
each level: left → right on the first level, right → left on the second,
then left → right again, and so on.

solve(levelOrder) receives the tree serialized level by level and returns an
array of arrays (one per level). An empty tree returns [].

Examples

solve([3, 9, 20, null, null, 15, 7])  → [[3], [20, 9], [15, 7]]
solve([1, 2, 3, 4, 5, 6, 7])          → [[1], [3, 2], [4, 5, 6, 7]]
solve([1])                            → [[1]]
solve([])                             → []

Keep the BFS itself in normal left-to-right order — only the output of every
other level is reversed.