Direct your coding agent, review the file it writes, and test it against evolving requirements. Each scenario has three stages and a limited prompt budget.
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Easy · Hash Map · Arrays
Find two numbers that add to a target and return their positions. Walk the list once, remembering each number seen: for two_sum([2, 7, 11, 15], 9), 9 - 2 = 7 is already seen, so the answer is [0, 1].
Medium · Sorting · Intervals
Fold overlapping time ranges into one clean cover. Sort by start, then sweep: [1, 3] and [2, 6] overlap into [1, 6], while [8, 10] and [15, 18] stand alone.
Medium · Sliding Window · Arrays
Guard an API: each request is allowed only while fewer than capacity already-allowed requests sit inside the window. With capacity 2 and window 5, requests at [0, 1, 2, 3] give [1, 1, 0, 0] — denied requests never consume quota.
Medium · Math · Simulation
Schedule waits between retries that grow exponentially but never pass a cap. With base 1, factor 2, cap 5 over 4 failures: waits are 1, 2, 4, then 8 capped to 5 — [1, 2, 4, 5].
Medium · Sorting · Intervals
Book the fewest rooms that fit every meeting. Sort starts and ends, then sweep: meetings [9, 10] and [10, 11] touch but never overlap, so 1 room is enough — an ending meeting frees its room first.
Medium · Hash Map · Strings
Read a failing service log and name the most frequent error code. For [500, 404, 500] the answer is 500 (seen twice); ties break to the smallest code string, so [404, 500] returns 404.
Medium · Greedy · Arrays
Play warehouse keeper: walk each order in turn, fulfill it if stock remains, and never let a skipped order block a later one. With stock 6 and orders [4, 2, 5]: 4 fits (2 left), 2 fits (0 left), 5 does not — [1, 1, 0].
Medium · Filtering · Arrays
Scrub a signup sheet, keeping rows with age 18-99 and ZIP 10000-99999, bounds inclusive. For ages [25, 17, 30] and ZIPs [12345, 90210, 999]: row 0 is valid, row 1's age fails, row 2's ZIP fails — [0].
Hard · Sweep Line · Intervals
Audit a room calendar with a capacity limit and flag every meeting past capacity, in sweep order. With capacity 1 and meetings [9, 11], [10, 12]: the second starts while the first still runs, so index 1 is flagged.