Sort Colors (Dutch National Flag): three-way partition in one pass.
Sorting an array of three values in one pass probes the three-pointer partition (Dutch National Flag). Interviewers look for the low/mid/high invariant and knowing when not to advance mid. The answer follows.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Sorting an array of three values in one pass probes the three-pointer partition (Dutch National Flag). Interviewers look for the low/mid/high invariant and knowing when not to advance mid. The answer follows.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
