- LeetCode 5 – Longest Palindromic Substring
Counting substrings is O(n³) and the DP table costs O(n²) memory. Neither is the answer you want: a string has only 2n − 1 centres, and expanding around each one is O(n²) time in O(1) space and about fifteen lines. The even-length centre everyone forgets, the hi − lo − 1 off-by-one, the interval DP for when you need it, and where Manacher's linear algorithm fits.
- LeetCode 2 – Add Two Numbers
Long addition wearing a linked list costume. Reverse digit order is a gift, not an obstacle — it points the lists the same way the carry travels. The dummy head that removes the first-node special case, the carry in the loop condition that gives 999 + 1 its fourth node, and why converting to an integer overflows on the real test cases. Java and Python, plus the forward-order follow-up.
- LeetCode 1 – Two Sum
The first problem on LeetCode, and still the most common phone-screen warm-up. It is not a test of whether you can find two numbers — it is a test of whether you reach for a hash map the moment you catch yourself writing a nested loop. One pass, look up before you insert, and the target = 2 × nums[i] case that lets an element pair with itself. Java and Python, plus the sorted two-pointer variant that 3Sum is built on.
- System Design – Where to Start
What system design actually is, why the question has no single right answer, and the four-step framework that keeps an hour from wandering: scope the requirements, estimate the load, sketch a high-level design, then go deep on the part that matters. What an interviewer is really scoring, the mistakes that sink candidates who know the material, and how the rest of this track is ordered.
- Elasticsearch – What to Do Before You Go to Production
The list, in the order it bites. Shard sizing and why more shards is usually the wrong instinct, replicas and what green actually promises, JVM heap and the 50% rule, security beyond turning it on — an API key scoped to one index rather than the elastic superuser every guide leaves you using — index lifecycle management, what to monitor, and the failure modes to rehearse before they happen.