- Designing a Customer Data Platform
CRM, Billing and Support each think they own the customer, they disagree, and they arrive out of order. The design that produces one profile and can still explain why it chose every value — field-level observations, a pure survivorship function, identity resolution with reversible merges, and a lineage endpoint that shows what lost.
- MySQL – Transactions
Making several statements succeed or fail together. START TRANSACTION, COMMIT and ROLLBACK, autocommit and why it is on by default, savepoints, the four isolation levels and what each one lets through, why MySQL's default is REPEATABLE READ when most databases use READ COMMITTED, SELECT ... FOR UPDATE, and the DDL statement that silently commits your transaction out from under you.
- LeetCode 81 – Search in Rotated Sorted Array II
Problem 33 with duplicates, which looks like a one-line change and is not. Two arrays with the pivot in different halves can present identical evidence at every point the algorithm may look, so no decision is right for both. The worst case is O(n) — and that bound is on the problem, not on your approach.
- LeetCode 78 – Subsets
The cleanest backtracking problem there is, with one structural difference worth spotting: every node of the recursion tree is an answer, not just the leaves, so the base case disappears. Plus the two bugs — i + 1 rather than start + 1, and the copy without which all 2^n entries alias one list.
- MongoDB – CRUD Operations
insertOne through deleteMany, upserts and bulk writes, and why an update should touch one field rather than rewrite a document.