Design an Elevator System
An office tower has a bank of elevators serving the same floors. People on a floor press up or down (a hall call); people inside a car pr...
Free interview preparation
Prepare for coding, SQL and system design interviews.
Solve real interview questions in a browser workspace with an editor, test cases, hints and worked solutions. Free to use, with no account and nothing to install.
New here? Follow the study plan
- 169Coding and DSA problems
- 70SQL problems
- 70System design and LLD questions
- 14Languages in the editor
Practice areas
One workspace for each interview round
Each round is set up the way it is run in a real interview.
Coding and DSA
Problems grouped into 17 patterns, from arrays to dynamic programming. Python, JavaScript and TypeScript are judged automatically.
SQL
Queries run on a real database in your browser: joins, window functions, dates, funnels and retention, checked on hidden data.
System design
High-level designs from a URL shortener to payments, including AI systems such as RAG, inference serving and agents.
Low-level design
Object-oriented design with class diagrams and runnable code: parking lot, elevator, expense sharing, chess and more.
The workspace
Practise under interview conditions
Everything runs in your browser and your progress stays on your device.
Automatic judging
Code runs against visible and hidden tests, with time limits, and returns a verdict in seconds.
Mock interview timer
Timed sessions keep hints and solutions locked until time is up. Each problem keeps its own timer.
Hints before solutions
Hints come one at a time. The full solution and likely follow-up questions are there when you want them.
Design whiteboard
Drag-and-drop architecture and UML boards, a capacity estimator and a design doc, with a reference design to compare.
Spaced review
Solved problems come back after 1, 3, 7, 14 and 30 days, so you can re-solve them from memory.
Progress and reminders
Streaks, a daily goal, an activity history and practice reminders, with export and import.
Role guides
What each role is asked
The rounds you can expect for each role, and what to practise for them.
Software
- Software EngineerDSA, system design, LLD
- Frontend EngineerJavaScript, UI, web performance
- Backend EngineerAPIs, databases, scaling
- Full Stack DeveloperEnd-to-end product rounds
- Mobile DeveloperiOS, Android, offline sync
- Game DeveloperC++, engines, maths, performance
- QA and SDETTest design, automation
Data and AI
- Machine Learning EngineerML coding, ML system design
- AI EngineerLLMs, RAG, agents, evaluation
- Forward Deployed EngineerAI deployments with customers
- Data ScientistSQL, statistics, ML
- Data EngineerSQL, pipelines, warehouses
- Data AnalystSQL, metrics, product sense
- MLOps EngineerTraining, serving, monitoring
Study plan
A plan that works for any role
Five steps, in order. Most people need four to eight weeks.
- Pick one languageGet fluent with its collections, strings and recursion.
- Learn the patternsWork through the Pattern Path one topic at a time, easy to hard.
- Add SQL and designMost interview loops include a data or design round.
- Run mock interviewsUse the timer with hints locked, and explain your thinking out loud.
- Review on scheduleRe-solve problems from memory when the review queue asks.
Articles
Latest interview guides
FAQ
Frequently asked questions
How the site works, and how to use it to prepare.
Is CodingInterview.net really free?
Yes. Every problem, hint, solution, the code editor, the SQL database, the system design whiteboard and the progress tracker are free, with no account and no paywall.
Which interview rounds can I practise here?
Coding and data structures & algorithms, SQL, high-level system design (including AI and LLM systems such as RAG, inference serving and AI agents) and low-level object-oriented design. Role guides cover what each role is asked, from software and data to embedded, robotics, DevOps and mobile.
Which programming languages are supported?
Python, JavaScript and TypeScript are judged automatically in your browser. Java, C++, C, C#, Go, Kotlin, Rust, Swift, Ruby, PHP and Scala run in the built-in online compiler.
Do I need to sign up or install anything?
No. Everything runs in your browser. Your progress, code and notes are saved privately on your device, and you can export a backup from the progress page.
How should I prepare for a coding interview in 4 to 8 weeks?
Pick one language, then work through the Pattern Path one technique at a time, two to four problems a day. Add a SQL or system design session twice a week, do a timed mock interview every weekend, and clear your review queue daily.
Is this useful for interviews at any company, in any country?
Yes. The questions cover the patterns and design topics that technical interviews use worldwide, from large tech companies to startups, and everything is written in clear international English.
Start with one problem today.
Pick a round, solve one problem, and let the review queue bring it back when it is due.