Representative Union Find problems
Find the length of the longest consecutive elements sequence in an unsorted array using array scanning and hash lookup.
Open problem page#130 Surrounded RegionsTransform the matrix in-place by marking regions surrounded by 'X' as 'X', while keeping border-adjacent 'O's intact.
Open problem page#200 Number of IslandsCount the number of distinct islands in a binary grid using array traversal combined with depth-first search exploration techniques.
Open problem page#399 Evaluate DivisionCompute the results of division queries from given equations using graph traversal and depth-first search efficiently.
Open problem page#547 Number of ProvincesSolve Number of Provinces by scanning the adjacency matrix and launching DFS once per unvisited city component.
Open problem page#684 Redundant ConnectionIdentify and remove the redundant edge that causes a cycle in a graph starting as a tree.
Open problem page#685 Redundant Connection IIFind and remove the redundant connection in a directed graph that was originally a rooted tree.
Open problem page#695 Max Area of IslandFind the largest connected land area in a binary grid using array traversal and depth-first search efficiently.
Open problem page#721 Accounts MergeMerge accounts by connecting emails and returning each user's sorted email list using array scanning and hash lookup efficiently.
Open problem pageUnion Find interview questions usually collapse into a small set of repeatable moves. This page groups those GhostInterview problem pages together so you can stay inside one pattern family, compare representative questions, and practice explanations that transfer across multiple prompts. If you are trying to sharpen one interview weakness instead of browsing the entire library, start here.
Why Union Find Matters In Coding Interviews
Interviewers rarely care about the label alone. They care about whether you can recognize when the Union Find pattern applies, explain why it applies, and avoid the failure modes that usually show up under time pressure. That is why this page focuses on the family of problems rather than a single isolated example.
Pattern families that show up here
- Graph traversal with depth-first search
- Array scanning plus hash lookup
- Array plus Depth-First Search
Use this page to narrow the session
Start from this topic hub, choose one or two representative problems, and then open the detailed problem pages only after you know which version of the pattern you want to reinforce.
How To Practice This Topic
The best workflow is to keep the prep block narrow. Pick one easy or medium problem to confirm the base pattern, then one medium or hard problem to test whether the explanation still holds when the constraints tighten. Once the logic is stable, move into GhostInterview to rehearse the live explanation flow.
Focus on transfer, not title count
Solving more Union Find titles is less useful than understanding how the same move changes across examples. Try to describe what stays the same and what changes from one problem to the next.
Use the problem page when you want the exact breakdown
This topic hub is the discovery layer. The single-problem pages are where you get the answer-first summary, examples, constraints, approach, complexity, pitfalls, and solver CTA.
How GhostInterview Fits
GhostInterview works best after you know which pattern family you are inside. Use this page to choose the right Union Find problem, then use the solver to get the answer path, complexity framing, and follow-up support without switching into a generic study flow.
FAQ
What does the Union Find topic usually test in interviews?
Union Find questions usually test whether you can spot the underlying pattern quickly, explain the trade-offs, and move from intuition to implementation without losing clarity.
How should I use this Union Find topic page?
Use it as a pattern hub. Start with the representative problems on this page, compare the common approach, and then open the exact problem page when you want the full breakdown.
How many GhostInterview problem pages are in the Union Find group?
This topic page currently groups 74 indexed GhostInterview problem pages under the Union Find family.
Should I practice this topic before harder patterns?
That depends on your current gaps. If Union Find is already a weak spot in interviews, it is usually better to stabilize it first before moving into more advanced or less frequent patterns.
Where does GhostInterview fit once I pick a Union Find problem?
The topic page narrows the pattern family. The problem page gives the exact breakdown. GhostInterview is the solver layer when you need direct help with execution, complexity, and follow-up handling.
Stay close to the same reasoning family
1,672 overlapping problems
Open topic pageBreadth-First Search197 overlapping problems
Open topic pageDepth-First Search252 overlapping problems
Open topic pageGraph141 overlapping problems
Open topic pageMatrix221 overlapping problems
Open topic pageHash Table610 overlapping problems
Open topic pageNeed direct help once you choose a Union Find problem?
Use GhostInterview as the solver layer after you select the exact prompt. Capture the question, get the answer path and complexity framing, and keep the workflow inside the same topic family.