Practice problems

150 coding problems by topic and difficulty. Open one to read the statement and solve it in the browser; hints are there when you need them.

Practice problems
#	Problem	Difficulty	Topic
1	Two Sum Target Lookup	Easy	Arrays & Hashing
2	Reverse String In-Place	Easy	Strings & Two Pointers
3	Validate Binary Search Tree	Medium	Trees
4	Coin Change	Medium	Dynamic Programming
5	Longest Increasing Subsequence	Medium	Dynamic Programming
6	Container With Most Water	Medium	Two Pointers
7	Reverse Linked List	Easy	Linked Lists
8	Linked List Cycle Entry	Medium	Linked Lists
9	Lowest Common Ancestor of a Binary Tree	Medium	Trees
10	Evaluate Boolean Binary Tree	Easy	Trees
11	LRU Cache	Medium	Design
12	Design a Sliding Window Rate Limiter	Medium	Design
13	Design a URL Shortener	Medium	Design
14	Diffie-Hellman Key Agreement	Medium	Cryptography
15	Network Packet Filter Simulation	Medium	Network Security
16	Combine Two Tables	Easy	SQL
17	Consecutive Numbers	Medium	SQL
18	Print in Order	Easy	Thread Synchronization
19	Print FooBar Alternately	Medium	Thread Synchronization
20	Add Two Numbers	Medium	Linked Lists & Math
21	Median of Two Sorted Arrays	Hard	Binary Search & Divide and Conquer
22	Trapping Rain Water II	Hard	Heap & Breadth-First Search
23	Word Ladder II	Hard	Graph, BFS & Backtracking
24	Alien Dictionary	Hard	Graph & Topological Sort
25	Burst Balloons	Hard	Dynamic Programming & Divide and Conquer
26	Shortest Path Visiting All Nodes	Hard	Bit Manipulation & Graph Search
27	Build a Personal Profile Page	Beginner	HTML Basics
28	Create an Accessible Navigation Menu	Beginner	HTML Basics
29	HTML Forms with Validation	Easy	HTML Basics
30	CSS Box Model Mastery	Beginner	CSS Styling
31	Flexbox Layout Challenge	Easy	CSS Styling
32	CSS Grid Dashboard Layout	Medium	CSS Styling
33	Responsive Navigation with Media Queries	Easy	CSS Layout
34	CSS Animations & Transitions	Medium	CSS Layout
35	Mobile-First Landing Page	Medium	Responsive Design
36	DOM Manipulation: To-Do List	Beginner	JS Basics
37	Array Methods Practice	Easy	JS Basics
38	Destructuring & Spread Operator	Easy	ES6+
39	Async/Await Error Handling	Medium	ES6+
40	React Component Hierarchy	Beginner	React Basics
41	useState Counter with History	Easy	React Basics
42	useEffect Data Fetching	Medium	React Basics
43	useReducer Shopping Cart	Medium	State Management
44	Context API Theme Switcher	Medium	State Management
45	REST API with Express	Medium	Node Basics
46	JWT Authentication Flow	Hard	Authentication
47	SQL Queries for Blog Platform	Easy	SQL Basics
48	MongoDB CRUD Operations	Easy	MongoDB
49	E-Commerce Product Page	Hard	E-Commerce
50	Hello World & Input/Output	Beginner	Getting Started
51	Data Types Explorer	Beginner	Variables & Types
52	String Manipulation Mastery	Easy	Strings
53	FizzBuzz with a Twist	Beginner	Conditionals
54	Grade Calculator with Edge Cases	Easy	Conditionals
55	Pattern Printing with Nested Loops	Easy	Loops
56	List Comprehension Challenges	Medium	Lists & Tuples
57	Function Decorators: Call Logger	Medium	Functions
58	Lambda, Map, Filter, Reduce	Easy	Functions
59	List Operations: Remove Duplicates Preserving Order	Easy	Lists & Tuples
60	Dictionary Deep Merge	Medium	Dictionaries & Sets
61	Class: Bank Account System	Medium	Classes
62	Inheritance & Polymorphism: Shape Hierarchy	Medium	Classes
63	NumPy-Style Array Operations	Easy	Lists & Tuples
64	Pandas-Style Sales Analysis	Medium	Files & Modules
65	Web Scraper: Extract Article Links	Medium	Files & Modules
66	Calculator Engine for a Tkinter GUI	Medium	Functions
67	Types of Machine Learning	Beginner	What is ML
68	ML Pipeline Design	Easy	ML Pipeline
69	Linear Regression from Scratch	Medium	Regression
70	Logistic Regression Binary Classifier	Medium	Classification
71	Decision Tree by Hand	Medium	Decision Trees
72	K-Means Clustering Visualization	Medium	Clustering
73	PCA Dimensionality Reduction	Hard	Dimensionality Reduction
74	Neural Network with NumPy	Hard	Perceptrons
75	CNN Architecture Design	Hard	Convolutional NN
76	Transformer Attention Mechanism	Hard	Transformers
77	Prompt Engineering Challenge	Easy	Prompt Design
78	CIA Triad Analysis	Beginner	CIA Triad
79	Risk Assessment Matrix	Easy	Risk Assessment
80	Access Control Models	Easy	Access Control
81	Caesar Cipher Implementation	Beginner	Encryption Basics
82	AES Encryption with Python	Medium	Symmetric Crypto
83	RSA Key Generation & Signing	Hard	Asymmetric Crypto
84	Firewall Rule Design	Easy	Firewalls
85	SQL Injection Lab	Medium	Web Attacks
86	XSS Attack & Prevention	Medium	Web Attacks
87	OSINT Investigation	Easy	OSINT
88	Incident Response Plan	Medium	Incident Response
89	OSI Model Deep Dive	Beginner	OSI Model
90	TCP vs UDP Comparison	Easy	TCP/IP
91	TCP 3-Way Handshake Diagram	Easy	TCP/IP
92	Subnetting Practice Set	Medium	Subnetting
93	Routing Protocol Comparison	Medium	Routing Protocols
94	DNS Resolution Walkthrough	Easy	DNS
95	DHCP DORA Process	Easy	DHCP
96	Network Troubleshooting Scenario	Medium	Troubleshooting
97	SELECT Queries Practice	Beginner	SELECT
98	JOIN Types Mastery	Easy	JOINs
99	Aggregation & GROUP BY	Easy	Aggregation
100	ER Diagram Design: Library System	Medium	ER Diagrams
101	Normalization Practice	Medium	Normalization
102	Index Strategy Design	Hard	Indexing
103	MongoDB Aggregation Pipeline	Medium	CRUD Operations
104	Process States & Transitions	Beginner	Processes
105	CPU Scheduling Algorithms	Medium	CPU Scheduling
106	Virtual Memory & Page Table	Medium	Virtual Memory
107	Page Replacement Algorithms	Medium	Virtual Memory
108	Deadlock Detection	Hard	Deadlocks
109	Semaphore & Mutex Implementation	Hard	Semaphores
110	Agile vs Waterfall Comparison	Beginner	Waterfall
111	Scrum Ceremonies & Artifacts	Easy	Scrum
112	UML Class Diagram: ATM System	Medium	UML
113	System Design: URL Shortener	Hard	System Design
114	Unit Testing with Jest	Easy	Unit Testing
115	TDD: FizzBuzz Kata	Medium	TDD
116	Singleton & Factory Patterns	Medium	Creational
117	Observer & Strategy Patterns	Medium	Behavioral
118	Git Merge Conflict Resolution	Easy	Git Basics
119	Git Rebase vs Merge	Medium	Branching
120	Time Complexity Analysis	Beginner	Time Complexity
121	Fast I/O Optimization	Easy	Fast I/O
122	Modular Arithmetic & GCD	Medium	Number Theory
123	Sieve of Eratosthenes	Medium	Number Theory
124	Segment Tree with Lazy Propagation	Hard	Segment Trees
125	Fenwick Tree (BIT) Implementation	Medium	Fenwick Trees
126	KMP String Matching	Hard	String Algorithms
127	Two Sum Variants	Easy	Array Problems
128	LRU Cache Implementation	Medium	Array Problems
129	Binary Tree Traversals	Easy	Tree Problems
130	Design a Chat System	Hard	Design Principles
131	Design a Rate Limiter	Medium	Scalability
132	STAR Method Practice	Easy	STAR Method
133	Salary Negotiation Strategy	Easy	Salary Negotiation
134	Temperature Converter	Beginner	Variables & Types
135	Seconds to Clock Time	Beginner	Operators
136	Split the Bill	Easy	Operators
137	Name Initials	Beginner	Strings
138	Leap Year Check	Beginner	Conditionals
139	Cinema Ticket Price	Beginner	Conditionals
140	Triangle Classifier	Easy	Conditionals
141	Sum of Digits	Beginner	Loops
142	Collatz Steps	Easy	Loops
143	Run-Length Encoding	Easy	Loops
144	Password Strength Checker	Easy	Functions
145	ISBN-10 Validator	Easy	Functions
146	Second Largest Value	Easy	Lists & Tuples
147	Word Frequency Counter	Easy	Dictionaries & Sets
148	Common Elements	Easy	Dictionaries & Sets
149	Safe Integer Parser	Easy	Errors & Exceptions
150	CSV Grade Report	Easy	Files & Modules