Amazon·Software Engineer·Online Assessment (OA)·Intermediate

Intermediate

Jun 2026

Summary

Amazon OA for a SWE role. One algorithmic problem about minimizing replacements in an alternating drone delivery scenario. Pretty niche setup but boils down to a greedy or optimization problem once you strip away the story.

Questions Asked (1)

Given an array of shipment weights, two drones alternate deliveries and each has a carrying limit. Find the minimum number of shipments you need to replace so that some pair of drones can handle all deliveries in alternating order.

Algorithms & Data Structures

Author's notes

The drone story is just window dressing.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and define the objective: minimize replacements so that two drones with given limits can alternate deliveries covering all shipments. Then, model the problem as finding the best pair of drones (or limits) and computing the minimum replacements needed, likely using greedy or dynamic programming. Finally, discuss the algorithm's time and space complexity and potential edge cases.

Pro tip: Demonstrate a structured problem-solving approach: start by restating the problem in your own words, ask clarifying questions about input format and constraints, and then walk through a small example to validate your understanding before diving into the algorithm.

1. Clarify the problem

Restate the problem to ensure you understand: we have an array of shipment weights, two drones with carrying limits, and they alternate deliveries. We can replace some shipments (change their weights) to make it possible for the drones to handle all deliveries in alternating order. We need the minimum number of replacements.

2. Identify constraints and edge cases

Ask about the range of weights, number of shipments, and whether drone limits are fixed or can be chosen. Consider edge cases like empty array, single shipment, or when no replacements are needed.

3. Formulate a strategy

Think about how to assign shipments to drones in alternating order. Since the order is fixed, the drones must take turns: drone A handles positions 0,2,4,... and drone B handles 1,3,5,... (or vice versa). For a given pair of limits, we can compute the number of replacements needed by checking each shipment against the limit of the drone assigned to that position. We need to find the pair of limits (or choose which drone goes first) that minimizes replacements.

4. Develop an algorithm

If the drone limits are fixed, we can try both assignments (which drone starts) and count replacements. If we can choose limits, we might need to consider all possible pairs from the array or use binary search. A brute-force approach would be O(n^2) or O(n^3), but we can optimize by sorting or using prefix/suffix counts.

5. Analyze complexity and test

Discuss the time and space complexity of your solution. Walk through a small example to verify correctness. Mention potential optimizations and trade-offs.

Key Points to Mention

Clarify whether the drone limits are given or can be chosen; if given, the problem reduces to counting mismatches for two alternating assignments.
The alternating order means the drones are assigned to even and odd indices; we can try both assignments (drone A first or drone B first).
For a fixed assignment, the number of replacements is the count of shipments where weight > limit for that drone.
If limits are not fixed, we can consider all possible pairs of limits from the array or use binary search to find the minimum replacements.
Edge cases: empty array, all shipments fit, no possible solution (return -1 or infinity).
Time complexity: O(n) if limits are fixed, O(n log n) or O(n^2) if we need to search for optimal limits.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.