Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Amazon SWE interview with a string manipulation problem that had a follow-up on complexity tradeoffs. Pretty focused session, just the one algorithmic question but they pushed hard on the analysis side.

Questions Asked (1)

Given an alphanumeric string with mixed casing, find the index of the first character that appears exactly once. If no such character exists, return -1. Walk through an O(n) solution and compare it against a sorting-based approach in terms of time and space.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Got the linear scan approach pretty fast, two passes through the string, first to build a frequency map and second to find the first count-of-one.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then propose an O(n) solution using a hash map to count character frequencies and a second pass to find the first unique character. Compare this with a sorting-based approach, highlighting the trade-offs in time and space complexity.

Pro tip: Emphasize that the O(n) solution is optimal for time, but if memory is constrained, discuss alternatives like using a fixed-size array (for ASCII) or a bit vector, showing awareness of practical constraints.

1. Clarify requirements and edge cases

Ask about the character set (ASCII vs Unicode), string length limits, and expected behavior for empty strings or strings with no unique characters.

2. Propose O(n) solution

Use a hash map (or array for ASCII) to count frequencies in one pass, then iterate through the string to find the first character with count 1.

3. Analyze complexity

State that the O(n) solution runs in O(n) time and O(k) space, where k is the number of distinct characters (bounded by alphabet size).

4. Compare with sorting-based approach

Explain that sorting the string (O(n log n) time) and then scanning for unique characters is slower and may alter original indices, requiring additional data structures.

5. Summarize trade-offs and conclude

Conclude that the hash map approach is generally superior for time efficiency, while sorting might be considered if memory is extremely limited and O(n log n) time is acceptable.

Key Points to Mention

Time complexity: O(n) for hash map vs O(n log n) for sorting
Space complexity: O(k) for hash map (k distinct chars) vs O(n) for sorting (if copying) or O(1) if in-place but modifies input
Two-pass approach: first pass to count, second pass to find first unique
Use of fixed-size array for ASCII to achieve O(1) space
Edge cases: empty string, all characters repeated, Unicode characters
Stability: sorting loses original index information, requiring extra steps to track first occurrence

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