LeetCode

What it offers

• Problem archive: thousands of problems tagged by topic and difficulty, with an online judge in all common languages.
• Study plans: curated sequences such as LeetCode 75 (75 essential problems, aimed at one to three months of preparation) and Top Interview 150 (aimed at three months or more), plus topic plans for SQL, dynamic programming and others.
• Community: a discussion section and a solutions tab on every problem, where users post explanations in many languages. Some of the best free study guides live here.
• Contests: weekly and biweekly contests with a rating system, the closest free approximation of solving under time pressure.
• Premium: a paid subscription that adds company-tagged problem lists, extra problems, official editorial solutions for more problems and other features. Check the official Premium page for current prices.

What it is good at

Volume and feedback. When you know exactly what you need to practise, the archive has it, and the judge removes any doubt about whether your solution works, including edge cases you did not think of. Most curated lists (NeetCode 150, Grind 75, Blind 75, Sean Prashad's patterns) point to LeetCode problems.

Where it fails people

It has no opinion about what you should do next. Left alone with it, many people solve easy array problems for months and feel productive without improving at the problems interviews actually ask. Grinding hundreds of random problems is slower than learning patterns.

Is Premium worth it?

For most people, not until the final month before a specific company's interview, when company-tagged lists become useful. The free tier plus a curated list covers the vast majority of preparation.

How to use it well

• Bring a plan. Follow a curated list or one of LeetCode's own study plans rather than browsing.
• Time-box attempts. Around 20 to 30 minutes on a medium, then read a hint or the solution, understand it, and redo the problem a few days later.
• Use tags to drill weaknesses. If you keep failing interval or heap problems, filter by that tag for a week.
• Read the top community solution only after you have a working answer, and compare approaches and complexity.
• Try a few contests once you are comfortable with mediums, to practise under a clock.
• Explain your solution aloud as if in an interview; the judge only checks correctness, not communication.

Common questions

How many problems should I solve? There is no magic number. 150 to 250 well-understood problems across all major patterns is enough for most interviews; understanding matters more than count.

Easy, medium or hard? Interviews mostly ask mediums. Use easies to learn a pattern, then spend most of your time on mediums.

Best for: the repetition phase, once you already know what you are practising.