The Coupon / Category Round

One round in the Wayfair loop looks like nothing on LeetCode, and it is the one that quietly decides most outcomes. It runs on the HackerRank pair-programming portal and lasts about an hour. It is a single base problem with three follow-ups stacked on top, each building on the same scenario: coupons that apply across Wayfair's tree of product categories. The interviewer hands you the base, watches you talk through it, then keeps adding constraints until the room is asking for constant-time lookups. Candidates report drawing this exact problem across multiple loops, which makes it the most reliably Wayfair-specific artifact you can prepare for.

Strip off the retail dressing and it is a tree problem with one twist worth internalizing before anything else. A coupon is attached to a category, and it cascades down to every child of that category. So the coupons that apply to a given category are the ones attached to it or to any of its ancestors, all the way up to the root. Get that inversion and the base falls out in a handful of lines. The follow-ups are not new algorithms; they are about arranging the work so that the same lookup, repeated thousands of times, stays cheap.