Union Find LeetCode Problems: All 83, With Python Solutions
Every problem in this library that LeetCode tags Union Find — 83 in total, 66 of them with a complete Python solution, a worked example and the time and space complexity of the approach.
- 83 problems
- 1 Easy
- 44 Medium
- 38 Hard
How Union Find problems are solved
A tag names the subject, not the method. These pattern hubs cover the techniques that actually solve Union Find problems — each one explains the approach, gives a Python template and states its complexity.
- Union-Find — Merge groups and ask whether two things are connected, both in near-constant time.
Union Find problems by difficulty
Problems with a complete Python solution are listed first, then by ascending problem number.
Easy (1)
| # | Problem | Difficulty | Topics |
|---|---|---|---|
| 1971 | Find if Path Exists in Graph | Easy | Depth-First Search, Breadth-First Search, Union Find +1 |
Medium (44)
| # | Problem | Difficulty | Topics |
|---|---|---|---|
| 128 | Longest Consecutive Sequence | Medium | Union Find, Array, Hash Table |
| 130 | Surrounded Regions | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 200 | Number of Islands | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 399 | Evaluate Division | Medium | Depth-First Search, Breadth-First Search, Union Find +4 |
| 547 | Number of Provinces | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 684 | Redundant Connection | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 695 | Max Area of Island | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 721 | Accounts Merge | Medium | Depth-First Search, Breadth-First Search, Union Find +4 |
| 785 | Is Graph Bipartite? | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 886 | Possible Bipartition | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 947 | Most Stones Removed with Same Row or Column | Medium | Depth-First Search, Union Find, Graph +1 |
| 959 | Regions Cut By Slashes | Medium | Depth-First Search, Breadth-First Search, Union Find +3 |
| 990 | Satisfiability of Equality Equations | Medium | Union Find, Graph, Array +1 |
| 1020 | Number of Enclaves | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 1061 | Lexicographically Smallest Equivalent String | Medium | Union Find, String |
| 1202 | Smallest String With Swaps | Medium | Depth-First Search, Breadth-First Search, Union Find +4 |
| 1254 | Number of Closed Islands | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 1267 | Count Servers that Communicate | Medium | Depth-First Search, Breadth-First Search, Union Find +3 |
| 1319 | Number of Operations to Make Network Connected | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 1361 | Validate Binary Tree Nodes | Medium | Tree, Depth-First Search, Breadth-First Search +3 |
| 1391 | Check if There is a Valid Path in a Grid | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 1559 | Detect Cycles in 2D Grid | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 1584 | Min Cost to Connect All Points | Medium | Union Find, Graph, Array +1 |
| 1631 | Path With Minimum Effort | Medium | Depth-First Search, Breadth-First Search, Union Find +4 |
| 1722 | Minimize Hamming Distance After Swap Operations | Medium | Depth-First Search, Union Find, Array |
| 1905 | Count Sub Islands | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 2316 | Count Unreachable Pairs of Nodes in an Undirected Graph | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 2368 | Reachable Nodes With Restrictions | Medium | Tree, Depth-First Search, Breadth-First Search +4 |
| 2424 | Longest Uploaded Prefix | Medium | Union Find, Design, Binary Indexed Tree +5 |
| 2492 | Minimum Score of a Path Between Two Cities | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 2658 | Maximum Number of Fish in a Grid | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 2685 | Count the Number of Complete Components | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 2812 | Find the Safest Path in a Grid | Medium | Breadth-First Search, Union Find, Array +3 |
| 2948 | Make Lexicographically Smallest Array by Swapping Elements | Medium | Union Find, Array, Sorting |
| 261 | Graph Valid TreePremium | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 323 | Number of Connected Components in an Undirected GraphPremium | Medium | Depth-First Search, Breadth-First Search, Union Find +1 |
| 694 | Number of Distinct IslandsPremium | Medium | Depth-First Search, Breadth-First Search, Union Find +2 |
| 737 | Sentence Similarity IIPremium | Medium | Depth-First Search, Breadth-First Search, Union Find +3 |
| 1101 | The Earliest Moment When Everyone Become FriendsPremium | Medium | Union Find, Array, Sorting |
| 1102 | Path With Maximum Minimum ValuePremium | Medium | Depth-First Search, Breadth-First Search, Union Find +4 |
| 1135 | Connecting Cities With Minimum CostPremium | Medium | Union Find, Graph, Minimum Spanning Tree +1 |
| 1258 | Synonymous SentencesPremium | Medium | Sort, Union Find, Array +3 |
| 2782 | Number of Unique CategoriesPremium | Medium | Union Find, Counting, Interactive |
| 2852 | Sum of Remoteness of All CellsPremium | Medium | Depth-First Search, Breadth-First Search, Union Find +3 |
Hard (38)
| # | Problem | Difficulty | Topics |
|---|---|---|---|
| 352 | Data Stream as Disjoint Intervals | Hard | Union Find, Design, Hash Table +3 |
| 685 | Redundant Connection II | Hard | Depth-First Search, Breadth-First Search, Union Find +1 |
| 765 | Couples Holding Hands | Hard | Greedy, Depth-First Search, Breadth-First Search +2 |
| 778 | Swim in Rising Water | Hard | Depth-First Search, Breadth-First Search, Union Find +4 |
| 803 | Bricks Falling When Hit | Hard | Union Find, Array, Matrix |
| 827 | Making A Large Island | Hard | Depth-First Search, Breadth-First Search, Union Find +2 |
| 839 | Similar String Groups | Hard | Depth-First Search, Breadth-First Search, Union Find +3 |
| 924 | Minimize Malware Spread | Hard | Depth-First Search, Breadth-First Search, Union Find +3 |
| 928 | Minimize Malware Spread II | Hard | Depth-First Search, Breadth-First Search, Union Find +3 |
| 952 | Largest Component Size by Common Factor | Hard | Union Find, Array, Hash Table +2 |
| 1489 | Find Critical and Pseudo-Critical Edges in Minimum Spanning Tree | Hard | Union Find, Graph, Minimum Spanning Tree +2 |
| 1569 | Number of Ways to Reorder Array to Get Same BST | Hard | Tree, Union Find, Binary Search Tree +7 |
| 1579 | Remove Max Number of Edges to Keep Graph Fully Traversable | Hard | Union Find, Graph |
| 1627 | Graph Connectivity With Threshold | Hard | Union Find, Array, Math +1 |
| 1632 | Rank Transform of a Matrix | Hard | Union Find, Graph, Topological Sort +3 |
| 1697 | Checking Existence of Edge Length Limited Paths | Hard | Union Find, Graph, Array +2 |
| 1970 | Last Day Where You Can Still Cross | Hard | Depth-First Search, Breadth-First Search, Union Find +3 |
| 1998 | GCD Sort of an Array | Hard | Union Find, Array, Math +2 |
| 2003 | Smallest Missing Genetic Value in Each Subtree | Hard | Tree, Depth-First Search, Union Find +1 |
| 2076 | Process Restricted Friend Requests | Hard | Union Find, Graph |
| 2092 | Find All People With Secret | Hard | Depth-First Search, Breadth-First Search, Union Find +2 |
| 2157 | Groups of Strings | Hard | Bit Manipulation, Union Find, String |
| 2334 | Subarray With Elements Greater Than Varying Threshold | Hard | Stack, Union Find, Array +1 |
| 2382 | Maximum Segment Sum After Removals | Hard | Union Find, Array, Ordered Set +1 |
| 2421 | Number of Good Paths | Hard | Tree, Union Find, Graph +3 |
| 2493 | Divide Nodes Into the Maximum Number of Groups | Hard | Depth-First Search, Breadth-First Search, Union Find +1 |
| 2503 | Maximum Number of Points From Grid Queries | Hard | Breadth-First Search, Union Find, Array +4 |
| 2573 | Find the String with LCP | Hard | Greedy, Union Find, Array +3 |
| 2612 | Minimum Reverse Operations | Hard | Breadth-First Search, Union Find, Array +2 |
| 2617 | Minimum Number of Visited Cells in a Grid | Hard | Stack, Breadth-First Search, Union Find +5 |
| 2709 | Greatest Common Divisor Traversal | Hard | Union Find, Array, Math +1 |
| 305 | Number of Islands IIPremium | Hard | Union Find, Array, Hash Table |
| 711 | Number of Distinct Islands IIPremium | Hard | Depth-First Search, Breadth-First Search, Union Find +2 |
| 1168 | Optimize Water Distribution in a VillagePremium | Hard | Union Find, Graph, Minimum Spanning Tree +1 |
| 1724 | Checking Existence of Edge Length Limited Paths IIPremium | Hard | Union Find, Graph, Minimum Spanning Tree |
| 2204 | Distance to a Cycle in Undirected GraphPremium | Hard | Depth-First Search, Breadth-First Search, Union Find +1 |
| 2307 | Check for Contradictions in EquationsPremium | Hard | Depth-First Search, Union Find, Graph +1 |
| 2371 | Minimize Maximum Value in a GridPremium | Hard | Union Find, Graph, Topological Sort +3 |
Keep exploring
All LeetCode topicsEvery tag with a page of its own, with the number of problems behind each.LeetCode patternsThe techniques themselves, with a Python template and complexity for each.Curated study listsBlind 75, NeetCode 150 and three more, grouped by pattern.Easy problemsStart here when the data structure is new rather than the problem.
Other topics
- Array1,569
- String672
- Hash Table588
- Math485
- Dynamic Programming481
- Sorting392
- Greedy346
- Depth-First Search289
- Binary Search253
- Database249
- Tree225
- Breadth-First Search223
- Matrix216
- Two Pointers201
- Bit Manipulation194
- Binary Tree174
- Heap (Priority Queue)163
- Prefix Sum157
- Stack157
- Simulation144
- Graph138
- Counting126
- Design122
- Sliding Window116
When the Union Find problem arrives live
Stealth Interview is a desktop app for macOS and Windows. It reads the coding problem off your screen, returns a working solution with a step-by-step explanation and its time and space complexity, and transcribes what the interviewer is saying — while staying invisible to screen sharing.