Implement `zigzag_level_order(values)` for a binary tree serialized as a zero-based heap array. For an existing node at index `i`, its children are at `2 * i + 1` and `2 * i + 2`. A `null` slot means no node, and an existing node never appears below a missing parent. Return one list per tree depth. Values on the root level appear left to right, values on the next level appear right to left, and the direction alternates at every following level. An empty serialization returns an empty list. For `values = [3, 9, 20, null, null, 15, 7]`, return `[[3], [20,9], [15,7]]`. Target `O(n)` time and `O(w)` working space, where `w` is the maximum number of nodes on one level, excluding the returned lists. ```hint Keep traversal and output direction separate Use ordinary breadth-first traversal to discover each level. A deque can place that level's values at the front or back according to the current direction. ``` ```hint Alternate once per completed level Change direction only after consuming the exact number of nodes that belonged to the current level. ``` ### Discussion Extensions - How would two stacks implement the same traversal? - Why can repeatedly inserting at the front of a dynamic array make a seemingly linear solution slower?