The Practical / Integration Round

You and the interviewer share a CoderPad, and the problem is a backend task with a payments skin on it. The round is collaborative. You talk through your thinking as you go, and looking things up on the open internet is allowed, because finding the right doc is part of the job. There is no silent whiteboard and no single hidden trick. What you are handed looks like real work, often a service that is already half built, and your job is to wire the next piece in without breaking what is there.

The recurring shape is money moving through code. A stream of charges from a seller's terminal. A card authorization that has to walk through capture, refund, and every way it can fail. A payment retry that must not bill the customer twice. The algorithm underneath is usually a recognizable medium. The difficulty lives in the second part the interviewer adds once your first version works, and in the edge cases money carries with it: rounding, duplicates, invalid states, a delivery that arrives twice. Get a working version standing, then take the layered follow-up gracefully.