For students and working engineers
Stop watching tutorials. Start getting good.
Placements are coming. Learn the patterns interviewers actually test, and prove you understand them instead of memorising them.
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The video course
Lecture 14 of 212: Hashing, part 3
- Someone else solves it while you watch.
- You never commit to a guess, so you're never wrong.
- “Completed” means you reached the end of the video.
A Cruxion lesson
Step 1 · Observe
Watch it fail, then find the idea
Crux:
- +You make the call before the answer appears.
- +Wrong guesses are the point. Crux notes them.
- +“Mastered” means you wrote it, fixed it and defended it.
8
stages to mastery
3
languages: Python, C++, Java
0
videos to sit through
Two Sum
Return the indices of the two numbers that add up to target.
nums target = 10
0
1
2
3
4
seen value → index
empty
Frame: read the constraints
n can reach 10,000. Checking every pair is about 50 million checks. The constraint is quietly telling you: one pass.
Step 1 of 12
Live demo · The sampling dial
next-token probabilitiesThe interview went ___
The usual range. Mostly sensible, with a little variety.
The LLM Engineer
Apprentice · How the model actually works
- How LLMs actually think
- Embeddings as a coordinate system for meaning
- Prompts as programs
Engineer · Make it know your data
- Retrieval-augmented generation, done right
- Supervised fine-tuning with LoRA and QLoRA
- Preference alignment: RLHF and DPO
Architect · Ship it and keep it honest
- Evaluation, and LLM-as-judge
- Inference at scale: KV cache, quantization
- LLMOps: from a script to a live service
p99 latency
38ms
Database load
22%
All green. The cache is answering nine reads out of ten.
High-level design
Scaling, caching, queues and consistency on request-flow diagrams you can break and fix.
Low-level design
Classes, interfaces and patterns, taught by extending real code until the design holds or cracks.
What you master in DSA carries straight into both.
Streaks that forgive one bad day
Miss a day and a banked freeze covers it automatically. No button, no guilt.
Wednesday: freeze used automatically. Streak: 6 days.
Mastery you actually earned
✓ MasteredA problem counts only once all eight stages pass. So the tick means something.
Frame
Observe
Predict
Control
Implement
Debug
Defend
Transfer
Two Sum · all eight stages cleared
Crux notices the problem behind the problem
Not “time to practise!” nudges. Crux tracks the kind of mistake you keep making and brings you back to it.
Crux grows with you
Earn XP, level up, and watch your companion glow brighter.
Level 3 → 4
- easyTwo SumAmazonGoogleMicrosoftAdobe
- mediumTwo Sum on a Sorted ArrayAmazonMetaMicrosoft
- easyBalanced BracketsAmazonMicrosoftBloombergMeta
- easyFind the TargetGoogleMicrosoftAmazon
- mediumLongest Run of Unique CharactersAmazonGoogleBloombergMicrosoft
- easyMajority ElementAmazonGoogle
- easyStock Buy & SellWalmartSwiggyGoogle
- mediumKadane's AlgorithmMicrosoftMeta
- medium4SumAdobeOYOUber
- mediumNext PermutationUberGoldman SachsAdobe
- mediumMerge Overlapping IntervalsGoogle
- mediumWord SearchOlaGoldman SachsGoogle
- hardCount InversionsGoogleAmazonSalesforce
- hardSliding Window MaximumFlipkartGoogleMicrosoft
Company names are trademarks of their owners, shown only to indicate where a question has been reported. No affiliation implied.
Sign up with Google and pick DSA or AI. Your first lesson takes about 16 minutes, and I'll be right there for it.