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Coding Practice

Solve algorithmic problems to earn knowledge and unlock badges.

Title	Difficulty ↑	Knowledge
Bubble Sort an Array		Bubble Sort
Merge Two Sorted Arrays		Merge Sort Insertion Sort
Find First and Last Position		Linear Search
Nth Fibonacci Number		Fibonacci
Reverse a Singly Linked List		Singly Linked List
Valid Parentheses		Stack
Two Sum Using Hash Map		Hash Table with Chaining
First Unique Character in a String		Hash Table with Open Addressing
Skip List Optimal Level Count		Skip List
Split a Multi-Responsibility Function		Single Responsibility Principle
Extensible Area Calculator		Open/Closed Principle
Device Capability Router		Interface Segregation Principle
Pluggable Notification System		Dependency Inversion Principle
Eliminate Discount Duplication		DRY - Don't Repeat Yourself
Simplify a Validator		KISS - Keep It Simple, Stupid
Build Only What You Need		YAGNI - You Aren't Gonna Need It
Guard Clause Validator		Fail Fast
Apply Sensible Defaults		Convention Over Configuration
Class Registry		Classes & Objects
Pure Calculator		Pure Functions
Print in Order		Async/Await Pattern
Parse URL		HTTP/HTTPS REST API
SQL SELECT Simulator		Tables, Rows & Columns
Two Sum II - Sorted Array		Two Pointers
Maximum Sum Subarray of Size K		Sliding Window
Power of N		Divide and Conquer Recursion
Move Zeroes		Array Operations
GCD of Array		GCD and LCM
Pascal's Triangle		Combinatorics
Insertion Sort a Linked List		Insertion Sort
Sort Colors (Dutch National Flag)		Selection Sort
Relative Sort Array		Counting Sort
Path Exists in Graph (DFS)		Depth-First Search
Validate Binary Search Tree		Binary Search Tree
Lowest Common Ancestor in BST		Binary Search Tree
Find K Largest Elements		Binary Heap
One Edit Away		Edit Distance
House Robber		House Robber
Detect Cycle in Linked List		Singly Linked List
Reverse a Doubly Linked List		Doubly Linked List
Doubly Linked List Palindrome Check		Doubly Linked List
Split Circular Linked List		Circular Linked List
Evaluate Reverse Polish Notation		Stack
Implement Queue Using Stacks		Queue
Implement Deque		Deque
Bloom Filter False Positive Rate		Bloom Filter
Reverse a String In-Place		Stack
Check If String Is Palindrome		Stack
Rotate Array by K Positions		Stack
Two Sum in Sorted Array		Stack
Check Balanced Brackets		Stack
Singleton Logger		Singleton
Shape Factory		Factory Method
Temperature Adapter		Adapter
Text Formatter Decorator		Decorator
Order Processing Facade		Facade
Character Flyweight Counter		Flyweight
Caching Proxy		Proxy
Notification Factory		Factory Method
Safe Type Substitution		Liskov Substitution Principle
Create a Reusable Formatter		DRY - Don't Repeat Yourself
Compose Entity Abilities		Composition Over Inheritance
Flatten Nested Access		Law of Demeter
Infer from Convention		Convention Over Configuration
Secure Wallet		Encapsulation
Shape Hierarchy		Inheritance
Animal Sounds		Polymorphism
Immutable Update		Immutability
Counter Factory		Closures
Collection Ops		Map, Filter, Reduce
Event Emitter		Event Loop
Async Parallel Limit		Async/Await Pattern Thread Pool
Thread-Safe Counter		Mutex & Locks Race Conditions
IP Address Validator		TCP vs UDP
Inner Join Implementation		SQL Joins
Generate Subsets		Backtracking
Next Greater Element		Monotonic Stack
Activity Selection		Greedy Algorithms
Rotate Array		Array Operations
Reverse Words in a String		String Manipulation
Count Primes		Sieve of Eratosthenes
Two Sum		Hash TablesArray Operations
Rotate Array		Array Operations
Move Zeroes		Two Pointers Array Operations
Valid Palindrome II		Two Pointers String Manipulation
Merge Two Sorted Lists		Linked ListsSortingTwo Pointers
Linked List Cycle Detection		Linked ListsTwo Pointers
Maximum Depth of Binary Tree		Binary TreesRecursion
Climbing Stairs		Dynamic Programming
Two Sum (Hash Map Approach)		Hash Tables
Valid Anagram		Hash TablesSorting
First Unique Character in a String		Hash TablesString Manipulation
Quick Sort an Array		Quick Sort
Merge Sort an Array		Merge Sort
Find Peak Element		Binary Search
Shortest Path in Unweighted Graph		Breadth-First Search
Kth Smallest Element in BST		Binary Search Tree
Check if Tree is Height-Balanced		AVL Tree
Autocomplete with Trie		Trie
Shortest Path in Weighted Graph		Dijkstra's Algorithm
Network Delay Time		Dijkstra's Algorithm
Connect All Cities with Minimum Cost		Prim's MST
Course Schedule Order		Topological Sort
Count Connected Components		Connected Components
Longest Common Subsequence		Longest Common Subsequence
Coin Change - Minimum Coins		Coin Change
Coin Change - Number of Ways		Coin Change
Longest Increasing Subsequence		Longest Increasing Subsequence
Josephus Problem		Circular Linked List
Min Stack - Get Minimum in O(1)		Stack
Group Anagrams		Hash Table with Chaining
Number of Connected Components		Union-Find / Disjoint Set
Maximum Subarray Sum		Stack
Next Greater Element		Stack
Circular Queue Operations		Queue
Task Scheduler with Cooldown		Binary Heap Queue
Kth Smallest Using Min Heap		Binary Heap
UI Theme Factory		Abstract Factory
SQL Query Builder		Builder
Prototype Object Cloner		Prototype
Shape Renderer Bridge		Bridge
File System Size Calculator		Composite
Nested Array Flatten Iterator		Iterator
Traffic Light State Machine		State
Configurable Sorter Strategy		Strategy
Pricing Strategy		Strategy
Org Chart Salary Total		Composite
Build a Processing Pipeline		Separation of Concerns
Data Store		Abstraction
Employee Bonus		Method Overriding
Function Transformer		Higher-Order Functions
Curried Math		Currying
Recursive Flatten		Recursion in FP
Rate Limiter (Token Bucket)		Semaphores
Producer-Consumer Queue		Producer-Consumer Problem Mutex & Locks
Task Scheduler with Cooldown		Concurrency vs Parallelism
HTTP Status Router		HTTP/HTTPS REST API
DNS Cache with TTL		DNS
Normalize a Table		Normalization
Container With Most Water		Two Pointers
Longest Substring Without Repeating Characters		Sliding Window
Subarray Sum Equals K (Prefix Sum)		Prefix Sum
Generate Permutations		Backtracking
Merge Intervals		Interval Problems
Generate Parentheses		Backtracking Recursion
Spiral Matrix		Matrix Traversal Two-Dimensional Arrays
Container With Most Water		Two Pointers Array Operations
Product of Array Except Self		Array Operations Prefix Sum
Longest Substring Without Repeating Characters		Sliding WindowHash Tables
Group Anagrams		Hash TablesSortingString Manipulation
String to Integer (atoi)		String Manipulation
Add Two Numbers (Array Representation)		Linked ListsArray Operations
Remove Nth From End of List		Linked ListsTwo Pointers
Intersection of Two Linked Lists		Linked ListsTwo Pointers
Binary Tree Level Order Traversal		Binary TreesBreadth-First Search
House Robber		Dynamic Programming
Unique Paths		Dynamic Programming
Top K Frequent Elements		Hash TablesSortingHeap
Min Stack		Stacks
Evaluate Reverse Polish Notation		Stacks
Daily Temperatures		StacksArray Operations
Find Minimum in Rotated Sorted Array		Binary Search
Search a 2D Matrix		Binary Search
Jump Game		GreedyDynamic Programming
Subsets		Backtracking Recursion
Heap Sort an Array		Heap Sort
Top K Frequent Elements		Heap Sort
Number of Islands		Depth-First Search
Word Search with Wildcards		Trie
Cheapest Flights Within K Stops		Dijkstra's Algorithm
Shortest Path with Negative Weights		Bellman-Ford
All-Pairs Shortest Paths		Floyd-Warshall
Minimum Spanning Tree Cost		Kruskal's MST
MST Using Prim's Algorithm		Prim's MST
0/1 Knapsack		0/1 Knapsack
Print Longest Common Subsequence		Longest Common Subsequence
Coin Change - Reconstruct Solution		Coin Change
Edit Distance (Levenshtein)		Edit Distance
Subarray Sum Equals K		Hash Table with Open Addressing
Redundant Connection		Union-Find / Disjoint Set
Bloom Filter Membership Check		Bloom Filter
Skip List Search Simulation		Skip List
All Root-to-Leaf Paths		Depth-First Search Binary Search Tree
Diameter of Binary Tree		Depth-First Search Binary Search Tree
Check If Graph Is Bipartite		Breadth-First Search Depth-First Search
House Robber		Fibonacci
Decode Ways		Fibonacci
Support Ticket Chain		Chain of Responsibility
Text Editor with Undo		Command
Chatroom Mediator		Mediator
State Snapshot Manager		Memento
Event Emitter		Observer
Data Pipeline Template		Template Method
HTTP Middleware Decorator		Decorator
Vending Machine State		State
Notification System		Interfaces & Abstract Classes
Game Character		Association & Composition
Pipeline		Function Composition
Read-Write Lock		Readers-Writers Problem Mutex & Locks
Deadlock Detector		Deadlock
Packet Reassembly		TCP vs UDP OSI Model
Build a Simple Index		Indexing
Transaction Log Replay		ACID Properties Transactions
3Sum		Two PointersSorting
Maximum Product Subarray		Dynamic ProgrammingArray Operations
Next Permutation		Array OperationsSorting
Longest Palindromic Substring		Dynamic ProgrammingString Manipulation
Validate Binary Search Tree		Binary TreesBinary Search TreesRecursion
Lowest Common Ancestor of a Binary Tree		Binary TreesRecursion
Clone Graph		GraphsBreadth-First SearchHash Tables
Number of Islands		GraphsDepth-First Search Breadth-First Search
Course Schedule		GraphsTopological Sort
Word Search		GraphsBacktracking Depth-First Search
Surrounded Regions		GraphsDepth-First Search
Longest Increasing Subsequence		Dynamic ProgrammingBinary Search
Word Break		Dynamic ProgrammingString Manipulation
Decode Ways		Dynamic Programming
Search in Rotated Sorted Array		Binary Search
Generate Parentheses		Backtracking Recursion
Merge Intervals		Array OperationsSorting Algorithms
Spiral Matrix		Array Operations Matrix Traversal
Set Matrix Zeroes		Array Operations Matrix Traversal
Minimum Size Subarray Sum		Sliding Window Two Pointers
Kth Smallest Element in a BST		Binary Search TreesTree Traversal
Binary Tree Right Side View		Binary TreesTree TraversalBreadth-First Search
Binary Tree Zigzag Level Order Traversal		Binary TreesBreadth-First Search
Count Complete Tree Nodes		Binary TreesBinary Search
Fruit Into Baskets		Sliding WindowHash Tables
Max Consecutive Ones III		Sliding Window
Counting Bits		Bit ManipulationDynamic Programming
Reverse Bits		Bit Manipulation
Pow(x, n)		Math FundamentalsRecursion
Sqrt(x)		Binary SearchMath Fundamentals
Happy Number		Math FundamentalsHash Tables
Implement Trie (Prefix Tree)		TrieDesign Patterns
Random Pick with Weight		Binary SearchMath Fundamentals
Kth Largest Element (Quick Select)		Quick Sort
Search in Rotated Sorted Array		Binary Search
Detect Cycle in Directed Graph		Depth-First Search
Minimum Knight Moves		Breadth-First Search
AVL Insertion Balance Factors		AVL Tree
Range Sum Query with Updates		Segment Tree
Range Minimum Query		Segment Tree
City Reachability Within Distance Threshold		Floyd-Warshall
Unbounded Knapsack		0/1 Knapsack
Sliding Window Maximum		Queue
Serialize and Deserialize Binary Tree		Breadth-First Search Binary Search Tree
Shortest Bridge Between Two Islands		Breadth-First Search Depth-First Search
Word Ladder Transformation		Breadth-First Search
Longest Increasing Subsequence		Fibonacci 0/1 Knapsack
Partition Equal Subset Sum		0/1 Knapsack
Expression Tree Visitor		Visitor
Reactive Computed Values		Observer
Calculator with Undo/Redo		Command
Dining Philosophers		Deadlock Semaphores
Construct Binary Tree from Preorder and Inorder		Binary TreesRecursion
Sliding Window Maximum		StacksSliding Window
Insert Interval		Array Operations
Longest Consecutive Sequence		Hash TablesArray Operations
Copy List with Random Pointer		Linked List OperationsHash Tables
LRU Cache		Hash TablesLinked List OperationsDesign Patterns
Sort List		Linked List OperationsSorting AlgorithmsMerge Sort
Network Delay Time		Graph TraversalDijkstra
Accounts Merge		Graph TraversalUnion-Find / Disjoint Set
Evaluate Division		Graph TraversalBreadth-First Search
Coin Change 2		Dynamic Programming
Palindrome Partitioning		Dynamic ProgrammingBacktrackingString Algorithms
Longest Substring with At Most K Distinct Characters		Sliding WindowHash Tables
Single Number II		Bit Manipulation
Design Add and Search Words		TrieDesign PatternsBacktracking
Design Twitter		Design PatternsHeap Priority QueueHash Tables
Snapshot Array		Design PatternsBinary SearchHash Tables
Paint Houses		Dynamic Programming
Minimum Cost to Connect All Points		Minimum Spanning Tree
Cheapest Flights Within K Stops		Graph Traversal
Find the City With the Smallest Number of Neighbors at a Threshold Distance		Graph Traversal
Minimum Spanning Tree (Kruskal's)		Minimum Spanning Tree
Flatten Binary Tree to Linked List		Binary Tree
Populating Next Right Pointers in Each Node		Binary Tree
K Closest Points to Origin		Heap
Reorganize String		Heap
Task Scheduler		Heap
Number of Connected Components in Undirected Graph		Union-Find / Disjoint Set
Redundant Connection		Union-Find / Disjoint Set
Number of Operations to Make Network Connected		Union-Find / Disjoint Set
Combination Sum II		Backtracking
Permutations II		Backtracking
Remove K Digits		Monotonic Stack
Non-overlapping Intervals		Greedy
Meeting Rooms II		Greedy
Design HashMap		Hash Table
Implement Queue Using Stacks		Stack
Design Circular Deque		Queue
Count Inversions		Merge Sort
Maximum Gap		Radix Sort
Currency Arbitrage Detection		Bellman-Ford
Word Break II		Fibonacci Depth-First Search
Trapping Rain Water		Stack
Merge K Sorted Arrays		Binary Heap
Mini Calculator Interpreter		Interpreter
Minimum Window Substring		Sliding WindowHash Tables
Trapping Rain Water		Array Operations Two Pointers
First Missing Positive		Array Operations
Merge k Sorted Lists		Linked List OperationsHeap Priority Queue
Serialize and Deserialize Binary Tree		Binary TreesTree TraversalDesign Patterns
Word Ladder		Breadth-First SearchGraph Traversal
Reconstruct Itinerary		Graph TraversalDepth-First Search
Edit Distance		Dynamic ProgrammingString Algorithms
Longest Valid Parentheses		Dynamic ProgrammingStacks
Interleaving String		Dynamic ProgrammingString Algorithms
Dungeon Game		Dynamic Programming
Best Time to Buy and Sell Stock III		Dynamic Programming
Maximum Profit in Job Scheduling		Dynamic Programming
Swim in Rising Water		Graph Traversal
Vertical Order Traversal of a Binary Tree		Binary Tree
Recover Binary Search Tree		Binary Tree
Find Median from Data Stream		Heap
Merge K Sorted Arrays		Heap
Text Justification		String Manipulation
Most Stones Removed with Same Row or Column		Union-Find / Disjoint Set
132 Pattern		Monotonic Stack
Employee Free Time		Greedy
Alien Dictionary		Topological Sort
Median of Two Sorted Arrays		Binary Search
Reverse Nodes in k-Group		Linked List Operations
Binary Tree Maximum Path Sum		Binary TreesTree TraversalDynamic Programming
Alien Dictionary		Graph TraversalTopological Sort
Maximal Rectangle		Dynamic ProgrammingStacksMatrix Traversal
Burst Balloons		Dynamic Programming
Minimum Window Substring		Sliding WindowHash Tables
Substring with Concatenation of All Words		Sliding WindowHash Tables
Wildcard Matching		Dynamic Programming
Distinct Subsequences		Dynamic Programming
Best Time to Buy and Sell Stock IV		Dynamic Programming
Critical Connections in a Network		Graph Traversal
Binary Tree Cameras		Binary Tree
Palindrome Pairs		String Manipulation
Smallest Window Containing All Characters		String Manipulation
Longest Duplicate Substring		String Manipulation
Shortest Palindrome		String Manipulation
N-Queens		Backtracking
Sudoku Solver		Backtracking
Largest Rectangle in Histogram		Monotonic Stack
Design Skiplist		Skip List
Critical and Pseudo-Critical MST Edges		Kruskal's MST
Regular Expression Matching		Fibonacci Edit Distance
Regular Expression Matching		Dynamic ProgrammingString Algorithms
LFU Cache		Hash TablesLinked List OperationsDesign Patterns
Minimum Cost to Merge Stones		Dynamic Programming
Shortest Path Visiting All Nodes		Graph Traversal
Word Search II		Backtracking
Maximal Rectangle		Monotonic Stack

352 problems