Union Find LeetCode Problems: All 83,
With Python Solutions

Every problem in this library that LeetCode tags Union Find. 66 of them carry a complete Python solution, a worked example and the time and space complexity of the approach.

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Easy: 1
Medium: 44
Hard: 38

1 pattern

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-FindMerge groups and ask whether two things are connected, both in near-constant time.

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python

Please analyze this context and help me answer the question or solve the problem.

Technical interview · 45 minsolution.pyShared editor · Python 3Longest substring without repeatsMediumGiven a string s, return the length of the longest contiguous substring in which every character is unique.s = "abcabcbb"
→ 31def length_of_longest_substring(s: str) -> int:2 last_seen = {}3 left = best = 04 for right, ch in enumerate(s):5 if ch in last_seen and last_seen[ch] >= left:

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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python

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python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

90%Subscription

python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

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QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

Union Find problems by difficulty

Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (1)

Easy problems
No.	Problem	Topics
1971	Find if Path Exists in Graph	Depth-First Search, Breadth-First Search, Union Find +1

Medium (44)

Medium problems
No.	Problem	Topics
0128	Longest Consecutive Sequence	Union Find, Array, Hash Table
0130	Surrounded Regions	Depth-First Search, Breadth-First Search, Union Find +2
0200	Number of Islands	Depth-First Search, Breadth-First Search, Union Find +2
0399	Evaluate Division	Depth-First Search, Breadth-First Search, Union Find +4
0547	Number of Provinces	Depth-First Search, Breadth-First Search, Union Find +1
0684	Redundant Connection	Depth-First Search, Breadth-First Search, Union Find +1
0695	Max Area of Island	Depth-First Search, Breadth-First Search, Union Find +2
0721	Accounts Merge	Depth-First Search, Breadth-First Search, Union Find +4
0785	Is Graph Bipartite?	Depth-First Search, Breadth-First Search, Union Find +1
0886	Possible Bipartition	Depth-First Search, Breadth-First Search, Union Find +1
0947	Most Stones Removed with Same Row or Column	Depth-First Search, Union Find, Graph +1
0959	Regions Cut By Slashes	Depth-First Search, Breadth-First Search, Union Find +3
0990	Satisfiability of Equality Equations	Union Find, Graph, Array +1
1020	Number of Enclaves	Depth-First Search, Breadth-First Search, Union Find +2
1061	Lexicographically Smallest Equivalent String	Union Find, String
1202	Smallest String With Swaps	Depth-First Search, Breadth-First Search, Union Find +4
1254	Number of Closed Islands	Depth-First Search, Breadth-First Search, Union Find +2
1267	Count Servers that Communicate	Depth-First Search, Breadth-First Search, Union Find +3
1319	Number of Operations to Make Network Connected	Depth-First Search, Breadth-First Search, Union Find +1
1361	Validate Binary Tree Nodes	Tree, Depth-First Search, Breadth-First Search +3
1391	Check if There is a Valid Path in a Grid	Depth-First Search, Breadth-First Search, Union Find +2
1559	Detect Cycles in 2D Grid	Depth-First Search, Breadth-First Search, Union Find +2
1584	Min Cost to Connect All Points	Union Find, Graph, Array +1
1631	Path With Minimum Effort	Depth-First Search, Breadth-First Search, Union Find +4
1722	Minimize Hamming Distance After Swap Operations	Depth-First Search, Union Find, Array
1905	Count Sub Islands	Depth-First Search, Breadth-First Search, Union Find +2
2316	Count Unreachable Pairs of Nodes in an Undirected Graph	Depth-First Search, Breadth-First Search, Union Find +1
2368	Reachable Nodes With Restrictions	Tree, Depth-First Search, Breadth-First Search +4
2424	Longest Uploaded Prefix	Union Find, Design, Binary Indexed Tree +5
2492	Minimum Score of a Path Between Two Cities	Depth-First Search, Breadth-First Search, Union Find +1
2658	Maximum Number of Fish in a Grid	Depth-First Search, Breadth-First Search, Union Find +2
2685	Count the Number of Complete Components	Depth-First Search, Breadth-First Search, Union Find +1
2812	Find the Safest Path in a Grid	Breadth-First Search, Union Find, Array +3
2948	Make Lexicographically Smallest Array by Swapping Elements	Union Find, Array, Sorting
0261	Graph Valid TreePremium	Depth-First Search, Breadth-First Search, Union Find +1
0323	Number of Connected Components in an Undirected GraphPremium	Depth-First Search, Breadth-First Search, Union Find +1
0694	Number of Distinct IslandsPremium	Depth-First Search, Breadth-First Search, Union Find +2
0737	Sentence Similarity IIPremium	Depth-First Search, Breadth-First Search, Union Find +3
1101	The Earliest Moment When Everyone Become FriendsPremium	Union Find, Array, Sorting
1102	Path With Maximum Minimum ValuePremium	Depth-First Search, Breadth-First Search, Union Find +4
1135	Connecting Cities With Minimum CostPremium	Union Find, Graph, Minimum Spanning Tree +1
1258	Synonymous SentencesPremium	Sort, Union Find, Array +3
2782	Number of Unique CategoriesPremium	Union Find, Counting, Interactive
2852	Sum of Remoteness of All CellsPremium	Depth-First Search, Breadth-First Search, Union Find +3

Hard (38)

Hard problems
No.	Problem	Topics
0352	Data Stream as Disjoint Intervals	Union Find, Design, Hash Table +3
0685	Redundant Connection II	Depth-First Search, Breadth-First Search, Union Find +1
0765	Couples Holding Hands	Greedy, Depth-First Search, Breadth-First Search +2
0778	Swim in Rising Water	Depth-First Search, Breadth-First Search, Union Find +4
0803	Bricks Falling When Hit	Union Find, Array, Matrix
0827	Making A Large Island	Depth-First Search, Breadth-First Search, Union Find +2
0839	Similar String Groups	Depth-First Search, Breadth-First Search, Union Find +3
0924	Minimize Malware Spread	Depth-First Search, Breadth-First Search, Union Find +3
0928	Minimize Malware Spread II	Depth-First Search, Breadth-First Search, Union Find +3
0952	Largest Component Size by Common Factor	Union Find, Array, Hash Table +2
1489	Find Critical and Pseudo-Critical Edges in Minimum Spanning Tree	Union Find, Graph, Minimum Spanning Tree +2
1569	Number of Ways to Reorder Array to Get Same BST	Tree, Union Find, Binary Search Tree +7
1579	Remove Max Number of Edges to Keep Graph Fully Traversable	Union Find, Graph
1627	Graph Connectivity With Threshold	Union Find, Array, Math +1
1632	Rank Transform of a Matrix	Union Find, Graph, Topological Sort +3
1697	Checking Existence of Edge Length Limited Paths	Union Find, Graph, Array +2
1970	Last Day Where You Can Still Cross	Depth-First Search, Breadth-First Search, Union Find +3
1998	GCD Sort of an Array	Union Find, Array, Math +2
2003	Smallest Missing Genetic Value in Each Subtree	Tree, Depth-First Search, Union Find +1
2076	Process Restricted Friend Requests	Union Find, Graph
2092	Find All People With Secret	Depth-First Search, Breadth-First Search, Union Find +2
2157	Groups of Strings	Bit Manipulation, Union Find, String
2334	Subarray With Elements Greater Than Varying Threshold	Stack, Union Find, Array +1
2382	Maximum Segment Sum After Removals	Union Find, Array, Ordered Set +1
2421	Number of Good Paths	Tree, Union Find, Graph +3
2493	Divide Nodes Into the Maximum Number of Groups	Depth-First Search, Breadth-First Search, Union Find +1
2503	Maximum Number of Points From Grid Queries	Breadth-First Search, Union Find, Array +4
2573	Find the String with LCP	Greedy, Union Find, Array +3
2612	Minimum Reverse Operations	Breadth-First Search, Union Find, Array +2
2617	Minimum Number of Visited Cells in a Grid	Stack, Breadth-First Search, Union Find +5
2709	Greatest Common Divisor Traversal	Union Find, Array, Math +1
0305	Number of Islands IIPremium	Union Find, Array, Hash Table
0711	Number of Distinct Islands IIPremium	Depth-First Search, Breadth-First Search, Union Find +2
1168	Optimize Water Distribution in a VillagePremium	Union Find, Graph, Minimum Spanning Tree +1
1724	Checking Existence of Edge Length Limited Paths IIPremium	Union Find, Graph, Minimum Spanning Tree
2204	Distance to a Cycle in Undirected GraphPremium	Depth-First Search, Breadth-First Search, Union Find +1
2307	Check for Contradictions in EquationsPremium	Depth-First Search, Union Find, Graph +1
2371	Minimize Maximum Value in a GridPremium	Union Find, Graph, Topological Sort +3

How Union Find problems are solved

When the Union Find problem arrives live.

Approach

Approach

Approach

Union Find problems by difficulty

Easy (1)1

Medium (44)44

Hard (38)38

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Medium (44)

Hard (38)