Stock-Flavored DP & Intervals

Dynamic programming is lighter at Robinhood than at a FAANG. It still shows up, and when it does it is almost comically on-brand: it is literally about buying and selling stock. A Premium question family, I through IV, is the canonical case. A price series, a question about the most profit you can extract, and a cap on how many times you may trade. Robinhood barely has to reframe it, because the textbook problem already is the company's product. Where a generic shop relabels a knapsack as a backpack, here the prices are prices and the transactions are transactions.

That familiarity is its own trap. Four problems look like four problems, so candidates memorize four solutions: a min-tracker for the single transaction, a sum of positive deltas for unlimited, a hand-rolled four-variable sweep for at-most-two. Then the at-most-k version arrives and they freeze, because they never noticed the first three are special cases of the fourth. The whole family is one state machine. Learn it once as buy and sell states indexed by how many trades you've opened. I, II, III and the cooldown variant all drop out of the same loop with a single constant changed.

The Robinhood angle

Write the recurrence you can defend. Code is written and run on CoderPad, and a correct O(nk) sweep that returns the right profit on the examples carries the round. A space-optimized rolling-array version that drops a state and underprices a trade does not. The edge cases are where money problems are won here: an empty price series, a k of zero, a strictly falling market where the right answer is to sit out and bank zero. Name those and handle them before you reach for any optimization. The brokerage does not pay you for an elegant off-by-one.

RobinhoodPremium lesson · Coding round

The 2 Robinhood problems are below.

1. 1A Premium questionCoding · Hard · Frequently asked
2. 2A Premium questionCoding · Medium

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