- MySQL – Users, Privileges and Roles
Not connecting as root. CREATE USER and why `'app'@'%'` and `'app'@'localhost'` are two different accounts, GRANT at the database, table and column level, the principle of least privilege applied to an application account, REVOKE, roles in MySQL 8 and why they need activating, `mysql_native_password` versus `caching_sha2_password`, and reading SHOW GRANTS to audit what an account can do.
- LeetCode 124 – Binary Tree Maximum Path Sum
The hardest version of the pattern this track has been building toward since Diameter: the recursion returns one quantity and records another. A path that uses both children cannot also reach the parent, which is the geometric fact behind the whole solution. Twelve lines, and no debugging helps if the two are confused.
- LeetCode 122 – Best Time to Buy and Sell Stock II
A rare case where removing a constraint makes the problem easier. Every profit is a sum of consecutive daily deltas, so take every positive one — and that derivation is what turns a greedy that looks like cheating into one that is obviously optimal. Plus the two-state form that survives fees, cooldowns and caps.
- AWS – Lambda: Handlers, Cold Starts and Packaging
The handler signature, what actually lives in the execution context between invocations, and why that one fact decides where you open a database connection. Cold starts measured rather than feared, memory as the CPU dial it really is, layers versus a zip versus a container image, and the packaging trap that ships a Lambda which dies at import: compiled wheels built for your laptop instead of for Lambda.
- LeetCode 121 – Best Time to Buy and Sell Stock
Worth more than its Easy tag, because the reframing it teaches is the one behind Kadane's algorithm and most 1-D DP. The brute force asks which pair of days is best; the linear solution asks what the best buy was if I sell today — and that has a one-variable answer. Why a falling market returns 0, and why this is Maximum Subarray in disguise.