- MongoDB – Indexes and the ESR Rule
Single, compound, multikey, unique and partial indexes, the field order that makes them work, and how to read explain().
- LeetCode 169 – Majority Element
Boyer-Moore voting is four lines that look like they cannot be correct, and the counting argument is short enough to give out loud: every disagreement cancels a pair, and a strict majority cannot be exhausted. The algorithm does not find the majority — it eliminates everything that cannot be it.
- LeetCode 168 – Excel Sheet Column Title
Looks like base-26 and is not: Excel's digits run A to Z representing 1 to 26, with no symbol for zero. That single missing digit is the entire problem, the fix is one decrement inside the loop, and 26 vs 27 is where every wrong solution shows itself.
- LeetCode 160 – Intersection of Two Linked Lists
An O(1)-space solution that looks like sleight of hand: when a pointer runs off one list, restart it on the other. The trick is two lines and the reason is one equation — a + c + b = b + c + a, so both arrive together. Switch on null, not on the last node.
- AWS – Alexa Skills: Where They Actually Live
Short and honest: an Alexa skill is not an AWS service. You build it on the Amazon Developer portal with the Alexa Skills Kit, and the only AWS part is the Lambda function it invokes. What that split means for where you configure things, what the ASK CLI does that the AWS CLI cannot, and how the skill's endpoint is wired to a function ARN. If you came here for AWS, the Lambda post is the one you want.