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

Intermediate

Jun 2026

Summary

Amazon SWE coding round, one question about counting special substrings in a DNA string. Pretty clean problem once you see the pattern, but I fumbled around longer than I should have before the structure clicked.

Questions Asked (1)

Given a DNA sequence string, count all substrings that are either exactly 2 identical characters, or longer substrings where the first and last characters match and the middle portion consists of exactly one distinct character.

Algorithms & Data Structures

Author's notes

Took me a while to even parse what 'special' meant here.

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AI HintsAI Generated

Suggested Approach

Clarify the definition of valid substrings: exactly two identical characters (e.g., 'AA') or longer substrings where the first and last characters match and all middle characters are the same distinct character (e.g., 'ABA', 'ABBA'). Then design an algorithm that counts these efficiently, likely by iterating over possible middle characters and using runs of identical characters to compute combinations.

Pro tip: After presenting your solution, discuss how you would test it with edge cases like empty string, single character, and strings with all identical characters, and mention the time and space complexity trade-offs.

1. Clarify the problem

Restate the definition of valid substrings and confirm with the interviewer. Ask about constraints (e.g., string length, character set) and expected output (count only).

2. Identify patterns

Recognize that valid substrings consist of a run of identical middle characters flanked by two identical characters (which may be the same as the middle). For length 2, it's just two identical characters.

3. Design an efficient algorithm

Iterate through the string, identify runs of identical characters, and for each run, compute the number of valid substrings that can be formed using that run as the middle portion. Use combinatorics to count without enumerating all substrings.

4. Analyze complexity

Determine the time and space complexity of your approach. Aim for O(n) time and O(1) extra space if possible.

5. Test with examples

Walk through small examples (e.g., 'AA', 'ABA', 'ABBA', 'AABAA') to verify correctness. Consider edge cases like empty string, single character, and all identical characters.

Key Points to Mention

Definition of valid substrings: length 2 with identical characters, or length ≥3 with matching first and last characters and all middle characters identical.
Observation that valid substrings can be characterized by runs of identical characters.
Use of combinatorics to count substrings without explicit enumeration.
Time complexity: O(n) by scanning the string once and processing runs.
Space complexity: O(1) extra space if using run-length encoding on the fly.
Edge cases: empty string, single character, strings with all identical characters, and strings with alternating characters.

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