Binary Tree Level Order Traversal

Use BFS because it naturally visits a binary tree by depth. If root is null, return an empty list. Otherwise enqueue root. At the start of each outer iteration, save levelSize = queue.length; these are precisely the nodes already known to be on the current level. Remove exactly levelSize nodes, append their values to a new level array, and enqueue each nonnull child. After the inner loop, append the completed level. Capturing the size before processing avoids accidentally mixing children into the parent level. For a zigzag variant, reverse alternate level arrays or use a deque, but do not change traversal correctness. Test null, one node, skewed trees, and a complete tree. Every node is enqueued and dequeued once, so time is O(n). The queue can hold the widest level, giving O(w) auxiliary space and O(n) in the worst case.