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Tree problem, not the hardest thing in the world but you need to think carefully about when to cut a node and what to do with its children.
Use a post-order DFS to process children first, then decide whether to delete the current node. When deleting a node, add its non-null children as new roots to the result list; otherwise, return the node to its parent.
Pro tip: Clarify edge cases upfront, such as deleting the root or multiple consecutive deletions, and discuss how the algorithm handles them. This shows thoroughness and prevents bugs during implementation.
Restate the problem: given a binary tree and a set of values to delete, return the forest of remaining trees. Ask about edge cases like deleting the root, empty tree, or deleting all nodes.
Select post-order DFS because it processes children before the parent, allowing you to know if a node's children become new roots when the node is deleted.
Write a DFS function that returns the node if not deleted, or null if deleted. If deleted, add its non-null children to the result list.
After DFS, if the root is not deleted, add it to the result list. The result list now contains all roots of the forest.
State time and space complexity: O(n) time and O(h) space for recursion. Walk through a small example to verify correctness.
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