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I went straight for BFS, collected each level into its own list, then reversed at the end.
Use a breadth-first search (BFS) to perform a standard level-order traversal from the root, collecting nodes level by level. Then reverse the order of the levels to get the traversal from deepest to root. Alternatively, use depth-first search (DFS) to record nodes at each depth and then reverse the levels.
Pro tip: Clarify with the interviewer whether 'deepest level up to the root' means reversing the order of levels while keeping nodes within each level left-to-right, or also reversing nodes within each level. This shows attention to detail and avoids misinterpretation.
Confirm the expected output format: list of lists where each inner list contains node values at a given depth, ordered from deepest to root. Ask if nodes within each level should remain left-to-right.
Decide between BFS with level reversal or DFS with depth tracking. BFS is straightforward for level-order; DFS can be more memory-efficient for skewed trees.
For BFS: use a queue, process level by level, and append each level's values to a result list. For DFS: recursively traverse, passing depth, and append node values to a list at that depth.
After collecting all levels in top-down order, reverse the list of levels to achieve deepest-to-root order. If using DFS, you can also insert each level at the beginning of the result list.
State time complexity O(n) and space complexity O(n) for BFS (queue) or O(h) for DFS (recursion stack). Discuss edge cases: empty tree, single node, skewed tree.
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