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Store the path from p to the root.
Traverse the path from q to the root, the first common point of the two paths is the LCA.
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Parent pointers allow traversal from any node directly toward the root without needing a full tree search. This simplifies the problem and enables efficient approaches such as hash set ancestor tracking or the two-pointer method.
Yes, LCA variations are common in FAANG and other top tech interviews. This version specifically tests understanding of tree structures, parent pointers, and techniques like hash sets or two-pointer traversal.
A hash set is a simple and effective data structure for solving this problem. It stores all ancestors of one node while traversing upward, allowing quick detection when the second node reaches a shared ancestor.
The optimal approach uses a two-pointer technique similar to finding the intersection of two linked lists. Each pointer moves up the tree using parent pointers and switches starting nodes when reaching the root. This guarantees they meet at the lowest common ancestor with O(1) extra space.