Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Amazon SWE coding round, one algorithmic question, nothing fancy. The kind of problem that looks straightforward until you're actually writing it out under pressure.

Questions Asked (1)

Given an unsorted integer array, find the smallest missing positive integer.

Algorithms & Data Structures

Author's notes

I went straight for sorting, which works but isn't the answer they're fishing for.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then propose an O(n) time and O(1) space solution using cyclic sort or in-place hashing. Explain that the answer lies in the range [1, n+1], and use the array itself to mark presence of numbers by swapping or negating values.

Pro tip: Amazon interviewers value clean, efficient code and clear communication. Before coding, explicitly state the time and space complexity of your approach and discuss trade-offs with simpler solutions like sorting or using a hash set.

1. Clarify constraints and edge cases

Ask about array size, possible values (negative, zero, duplicates), and whether modifying the input is allowed. Discuss edge cases like empty array, all negatives, or all positives in sequence.

2. Outline naive approaches and their trade-offs

Mention sorting (O(n log n)) or using a hash set (O(n) space) as baselines, then explain why they are suboptimal for large inputs or space constraints.

3. Propose optimal in-place approach

Explain that the smallest missing positive must be in [1, n+1]. Use cyclic sort to place each positive integer x at index x-1 if 1 <= x <= n. Then scan for the first index where nums[i] != i+1.

4. Walk through an example and code

Trace the algorithm on a sample array like [3,4,-1,1] to demonstrate correctness. Write clean code with clear variable names and handle edge cases.

5. Analyze complexity and test

State that time complexity is O(n) because each element is swapped at most once, and space is O(1). Suggest testing with edge cases and verifying the result.

Key Points to Mention

The answer is always in the range [1, n+1], where n is the array length.
In-place hashing or cyclic sort avoids extra space and achieves O(n) time.
Handle duplicates and out-of-range values by ignoring them during swaps.
After rearrangement, the first index i where nums[i] != i+1 gives the missing positive.
If all positions are correct, the answer is n+1.
Discuss trade-offs: sorting is simpler but slower; hash set uses extra space.

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