- LeetCode 131 – Palindrome Partitioning
Backtracking with a filter, and the cleanest illustration of where the recording happens: Subsets records at every node, this records only when the string is fully consumed. Get that wrong and partial partitions land in the output. Plus the precomputed palindrome table for the repeated work.
- LeetCode 125 – Valid Palindrome
A two-pointer warm-up with one nasty trap: '0' and 'P' differ by exactly 32, so the popular case-insensitive shortcut says they match. The 32 gap is a fact about letters, and it stops meaning anything the moment digits are in scope. Also why each skip loop needs its own bound.
- 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.