- LeetCode 68 – Text Justification
No algorithmic difficulty and one of the highest failure rates on the list, because the spacing rules have four interacting cases and each is a silent off-by-one. Split packing from padding before typing, collapse the four cases into two, and remember the single-word line where gaps would be zero.
- MySQL – JSON Columns
MySQL's native JSON type, which is not just a string. Storing and validating JSON, reading values with `->` and `->>`, JSON_EXTRACT, JSON_UNQUOTE, JSON_SET, JSON_ARRAY and JSON_OBJECT, JSON_TABLE for turning a document into rows, indexing a JSON path with a generated column — the only way to index one — and the question worth asking first: should this be a column instead?
- AWS – ElastiCache: Redis in Front of Your Database
Cache-aside in fifteen lines, then the four questions that decide whether it helps: what you key on, what you invalidate, what happens on a miss storm, and what happens when Redis is down. Valkey and Redis OSS engines, cluster mode on versus off, why your client needs the configuration endpoint and not a node address, and TTL as the only invalidation strategy that never goes stale forever.
- MySQL – Server and Session Functions
The small functions that tell you where you are and what you are connected to. VERSION(), DATABASE(), USER() and CURRENT_USER() and why they differ, CONNECTION_ID(), @@variables for session and global settings, SHOW STATUS and SHOW VARIABLES, BENCHMARK() for a crude timing, and SLEEP() — useful for reproducing a lock wait on purpose.
- LeetCode 67 – Add Binary
Looks like string manipulation, is really about carries — and the parse-to-integer answer everyone writes first is exactly what the 10,000-character constraint rules out. Why the carry belongs in the loop condition, why you build and reverse instead of prepending, and XOR and AND as sum-without-carry and carry.