Linked Lists and Design

Snap runs its coding rounds live, and the pointer problems are where a working answer separates from a sketch. Linked lists and a Premium question are the classic bank here: reverse a list, merge two sorted ones, a Premium question stored as digit lists, and build a cache that evicts the least-recently-used entry. None of it is dressed up as a Snapchat feature. It is the standard medium-tier set, and the Snap texture is the grading, not the prompt: code that compiles and runs, delivered fast, with the boundary cases handled on the first pass.

One habit carries the whole cluster. Every one of these problems is a small exercise in rewiring next pointers without losing the rest of the list. Save the next node before you overwrite the link. Lean on a sentinel node so the head and the empty case stop being special. Get the save-then-rewire reflex and the dummy-head trick automatic, and reversal, merge, and the cache all collapse into the same few lines of pointer bookkeeping.

The Snap angle

Snap's product is a stream of messages that arrive, get read, and expire. The data structures in this section sit behind that behavior, and framing them that way reads well in the room. A chat is an ordered list of messages you splice and reorder. A Premium question is exactly how a client keeps the most recent conversations warm and lets stale ones fall out of memory. You do not need to force the metaphor. Still, saying that a merge is how two ordered message streams fold into one timeline, or that eviction is how ephemeral state ages out, shows you see the structure under the product. On a loop graded for speed and clean execution, get a correct version running against the examples first, then narrate the pointer moves as you tighten it.