Lowest Common Ancestor of a Binary Tree III
A medium-tier problem at 82% community acceptance, tagged with Hash Table, Two Pointers, Tree. Reported in interviews at Meta and 4 others.
Lowest Common Ancestor of a Binary Tree III is a medium-difficulty problem that shows up in Meta, LinkedIn, and Atlassian interviews. Unlike standard LCA problems, this one gives you parent pointers instead of a full tree structure, which changes the approach entirely. You won't solve it by traversing down; you'll traverse up. The 82% acceptance rate is misleading, that's because most candidates who attempt it already understand the two-pointer trick. If you haven't drilled parent-pointer traversal, this is the problem that catches you flat on a live assessment. StealthCoder surfaces a working solution instantly if the pattern doesn't click during screen share.
Companies that ask "Lowest Common Ancestor of a Binary Tree III"
Lowest Common Ancestor of a Binary Tree III is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Made for the engineer who has done the work but might still blank with a webcam pointed at him.
Get StealthCoderThe key insight is treating this like a linked-list problem disguised as a tree problem. Both nodes have parent pointers, so you walk them upward until they meet. The obvious approach, collecting all ancestors in a hash set, works but wastes space. The smarter approach uses two pointers: advance both nodes up simultaneously, but when one hits the root, restart it at the other node's starting point. After two passes, they converge at the LCA. This two-pointer collision trick eliminates the hash table entirely and is what interviewers actually want to see. The topics map directly: Hash Table is the brute force, Two Pointers is the optimal path, and Binary Tree logic is background noise here. Most candidates freeze because they expect traditional traversal and miss that parent pointers invert the whole strategy. If you blank during your live assessment, StealthCoder runs invisibly and solves it in seconds.
Pattern tags
You know the problem.
Make sure you actually pass it.
Lowest Common Ancestor of a Binary Tree III recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Made for the engineer who has done the work but might still blank with a webcam pointed at him. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Lowest Common Ancestor of a Binary Tree III interview FAQ
Is parent-pointer LCA still asked at FAANG?+
Yes. Meta and LinkedIn specifically ask this variant. It's less common than classic LCA but appears in medium-difficulty rounds. The parent pointer constraint forces a different mental model, which is exactly why it's valued. Not as frequent as two-sum, but definitely in the rotation.
What's the trick that most candidates miss?+
Treating it as a linked-list collision problem instead of a tree problem. You don't traverse the tree structure at all. Walk both pointers upward, and when one reaches null, restart it at the other node's start. After two cycles, they meet at the LCA. This avoids hash tables and beats the brute force.
Can you solve this with just a hash set?+
Yes, collect all ancestors of node A in a set, then walk node B upward until you find a match. It's O(h) space and O(h) time, but interviewers prefer the two-pointer approach because it's O(1) space. If you're stuck, the hash set is a safe fallback.
How does this relate to the Hash Table topic?+
Hash Table is the brute-force solution path. Many candidates default to it because it feels safe. The problem tags it because it's valid, but optimal solutions avoid it. Understanding when to skip a comfortable approach is part of what makes this medium-difficulty.
Why is the acceptance rate so high?+
People who attempt it usually know the pattern. Most solve it in 10 to 15 minutes once the two-pointer insight clicks. If you hit it cold on a live OA without drilling parent-pointer traversal, it feels much harder than 82% suggests.
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