Hi! I'm Aryansh Kurmi

A Codeforces Div. 2 D that looks brutal — replace a[i] with a[i-1] - a[i] + a[i+1] whenever its two neighbours share parity — turns out to be nothing but sorting. Here is the whole thought process: how the difference array collapses that formula into a single adjacent swap, why the parity gate becomes 'same parity', and why sorting each run ascending is provably optimal.

A system design walkthrough of recommendation at scale — candidate generation, ranking, embeddings, the cold start problem, and why it's really a two-stage funnel.

Idempotency, retries, state machines, backpressure, queue-based load levelling, and the failure modes nobody warns you about — explained without jargon.