Sliding Window & Strings

By most candidate accounts, one string problem surfaces more than any other in this band: a Premium question. It shows up on phone screens and again as an onsite warm-up that opens into a harder follow-up. If you drill a single item from this lesson, drill that one. The rest of the family travels with it. A Premium question, valid palindrome, valid parentheses, and the at-most-K-distinct counting variants all run on the same two ideas: a window that grows and shrinks, or two pointers walking a string.

None of this is Spotify-invented. It is the standard sliding-window and string spine you would drill for any generalist loop, and Spotify runs it straight, sometimes phrasing the prompt in playback or playlist terms. The honest read is that the company's real signature lives next door, in the production-debugging case study and the system-design rounds, not here. What earns this family a lesson is weight. Counting over event streams leads the coding mix, and strings with sliding windows sit right behind it, so the patterns below pay back well per hour.

The Spotify angle

A readable, narrated medium clears this round more reliably than a silent optimal answer. Interviewers reward four things you can show on purpose. Descriptive names like longest_distinct_run over single letters. A short out-loud comparison of two approaches before you commit. Edge cases you raise before being asked. And a few inline assertions that check your own code. One stated grading criterion is whether you can explain the window to a non-technical listener. In practice, narrate the invariant as you type. Say what the window holds, why expanding the right edge only ever adds, and why the left edge never walks backward. The window is easy to write and easy to write subtly wrong, and the talking is what separates the two.