The Implementation Round

Jane Street rarely hands you an algorithm to recall. It hands you a thing to build, watches you build it, then keeps changing what it has to do. Connect Four becomes Connect K. A cache that memoizes an expensive call has to start evicting. A tree you just built now has to prove a single leaf to someone who only holds its root. Same problem, new requirement, and your first version either bends to it or gets rewritten in front of the interviewer.

That is the software loop, and it is a different animal from the quant battery this guide covers elsewhere. No timed arithmetic here, no market-making game. Just a small number of long rounds, each built on one open-ended problem in your strongest language, graded on a clean first pass with your reasoning spoken out loud. This lesson works four of the build-and-extend problems candidates report most, each as a runnable exercise with the extension an interviewer tends to layer on next.