Update a Layer Tree Preserving Z-Order

Problem

Given a layer tree, implement operations: reorder a layer (preserve z-order), reparent a layer, or compute the front-to-back render order.

Example

layers tree -> move layer up one; recompute z-order

Constraints

• Preserve sibling order; handle reparent into descendant

What to look for

Call out time/space trade-offs and edge cases before being asked.