Union Find interview questions
52 union find problems tagged across recent interview reports. Drilled most heavily by doordash, tiktok, and snap.
Union Find is a data structure that tracks connected components in a graph and answers reachability queries in near-constant time. With 52 problems tagged across the pattern, it's a high-frequency ask at DoorDash (24 problems), TikTok (21), and Snap (20). Candidates often recognize it too late: the problem statement hides the connection, you're merging sets, validating paths, or counting islands. If a Union Find variant lands in your live assessment and you freeze, StealthCoder reads the problem and delivers the solution invisible to the proctor.
Most-asked union find problems
Showing top 50 of 52 union find problems by # companies asking.
You can't drill every union find variant before the assessment. StealthCoder runs invisibly during screen share and solves whichever variant they throw at you. No browser extension. No detection signature. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code.
Get StealthCoderUnion Find problems fall into three buckets: explicit graph connectivity (validate trees, count components), implicit grid connectivity (islands, bricks falling), and dynamic equivalence (accounts merging, evaluate division). The pattern's signature is a sequence of union and find operations where you need to answer "are two nodes connected?" or "how many disjoint sets exist?" Drilling accounts-merge, couples-holding-hands, and detect-cycles-in-2d-grid first builds your intuition for rank-and-path-compression optimizations. Harder variants like divide-nodes-into-the-maximum-number-of-groups layer bipartite graph logic on top. Most candidates underestimate the variant space. StealthCoder is your safety net when a problem variant you didn't practice lands live.
Companies that hire most on union find
52 union find problems.
You won't drill them all. Pass anyway.
Union Find is one of the patterns interviews actually filter on. Memorizing every variant in a week is a fantasy. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds, no matter which union find flavor lands in your live OA. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Union Find interview FAQ
How many Union Find problems should I drill before my assessment?+
Start with 8-12 problems across the three buckets: graph connectivity, grid problems, and dynamic equivalence. accounts-merge and couples-holding-hands are canonical. Then solve 2-3 hard variants. 52 problems exist; you won't hit all of them live, but pattern recognition compounds fast.
How do I recognize a Union Find problem in the wild?+
Look for "connected," "component," "merge," "equivalence," or "relationship" in the problem statement. If you're tracking which nodes belong to the same group and answering connectivity queries, it's likely Union Find. Grid island problems often hint at it implicitly.
Is Union Find the most important pattern for DoorDash and TikTok?+
It's top-tier. DoorDash has 24 Union Find problems in their assessment pool; TikTok has 21. But they also test trees, graphs, and dynamic programming heavily. Union Find is a high-value pattern to lock down, not the only one.
What's the hardest Union Find variant I should know?+
divide-nodes-into-the-maximum-number-of-groups requires Union Find plus bipartite graph coloring. distance-to-a-cycle-in-undirected-graph and graph-valid-tree also fuse Union Find with other graph concepts. Practice these last.
Do I need perfect rank compression and path compression to pass?+
Path compression alone (make every node point directly to root) passes most problems. Full rank-by-union optimization matters for very large datasets or strict time limits. Implement it clean, test it, move on.