← Amazon Interview Insights

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

Intermediate
May 2026

Summary

Got an Amazon OA with a greedy/sorting problem about making array elements unique by incrementing values at a given cost. Pretty standard competitive programming flavor but easy to mess up the implementation details.

Questions Asked (1)

Q1

Given an array of product sizes (with possible duplicates) and a corresponding cost array where cost[i] is the price to increment size[i] by 1, find the minimum total cost to make all sizes unique.

Algorithms & Data Structures
Author's notes

My first instinct was to sort by size and greedily resolve conflicts left to right, but the cost array complicates things because you want to increment the cheaper item, not just the one that comes later.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Sort the sizes while keeping track of their costs, then use a min-heap to efficiently assign each size to the next available unique value, always choosing the cheapest increment. Alternatively, use a greedy approach with a priority queue to process sizes in ascending order and resolve conflicts by incrementing the size with the smallest cost until all are unique.

Pro tip: Clarify whether sizes can only be incremented (not decremented) and whether the cost array corresponds to the original indices; this affects the algorithm choice. Also, discuss time/space complexity trade-offs between sorting and heap-based approaches.

1. Understand the problem and constraints

Confirm that sizes can only be increased, costs are per unit increment, and duplicates must be resolved. Ask about input size limits to guide algorithm selection.

2. Choose an algorithm

Consider sorting sizes with their costs, then using a min-heap to track the cheapest available increments. Alternatively, use a greedy approach with a priority queue to process sizes in order.

3. Implement the solution

Sort the pairs by size, then iterate through, maintaining a min-heap of costs for sizes that need to be incremented. For each duplicate, pop the smallest cost, increment the size, and push the cost back if still conflicting.

4. Analyze complexity and edge cases

Discuss time complexity (O(n log n) due to sorting and heap operations) and space complexity (O(n)). Handle edge cases like all sizes identical, large gaps, and negative costs (if allowed).

5. Test and validate

Walk through a small example to verify correctness, such as sizes = [2,2,2] and costs = [1,2,3], ensuring the total cost is minimized.

Key Points to Mention

  • Greedy strategy: always increment the size with the smallest cost among duplicates.
  • Use of a min-heap (priority queue) to efficiently retrieve the minimum cost.
  • Sorting sizes to process in ascending order and avoid unnecessary increments.
  • Time complexity: O(n log n) due to sorting and heap operations.
  • Space complexity: O(n) for storing pairs and heap.
  • Edge cases: all sizes equal, large number of duplicates, and potential integer overflow in cost sum.

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