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

IntermediatePrefer not to say

Apr 2026Remote

Summary

Amazon SWE online assessment, one coding problem about simulating a malware attack on a password string and finding the minimum time before it becomes unrecoverable. The problem statement was buried under a lot of flavor text which honestly slowed me down more than the actual algorithm did.

Questions Asked (1)

Given a password string, a permutation array defining the order in which characters get corrupted to '', and a threshold m, find the minimum number of seconds before the number of distinct substrings containing at least one '' reaches or exceeds m. Return 1 if the condition is already met before any corruption.

Algorithms & Data Structures

Author's notes

Spent way too long parsing the problem description because it's wrapped in this whole 'cyber intrusion' narrative.

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AI HintsAI Generated

Suggested Approach

Model the corruption process as a sequence of states, where each state is defined by the set of corrupted positions. For each state, efficiently compute the number of distinct substrings containing at least one '*', and find the earliest time when this count reaches m. Use a data structure like a suffix automaton or suffix array to count distinct substrings dynamically as characters are replaced.

Pro tip: Clarify the definition of 'distinct substrings'—whether they are distinct by content or by position—and confirm the threshold condition (≥ m) and the return value for the initial state. This shows attention to detail and avoids misinterpretation.

1. Clarify the problem

Restate the problem in your own words, confirm the meaning of 'distinct substrings' (content-wise), and verify the threshold condition and return value for the initial state.

2. Brute-force approach

Describe a naive solution: simulate each second, replace the character at the given index with '*', and count distinct substrings containing '*' by generating all substrings. Analyze its time complexity.

3. Optimize counting

Propose an efficient method to count distinct substrings containing '*' after each corruption, such as using a suffix automaton or suffix array with dynamic updates, or maintaining a set of substrings.

4. Handle initial condition

Check if the initial string already has at least m distinct substrings containing '*' (which is impossible unless m=0, but clarify). If m=0, return 1 immediately.

5. Iterate and return

Iterate through the permutation array, update the string, recompute the count, and return the current second when the count first reaches or exceeds m. If never reached, return -1 or as specified.

Key Points to Mention

Definition of distinct substrings: substrings are distinct if their string content differs, regardless of position.
Initial condition: before any corruption, there are no '*' characters, so the count is 0. If m=0, return 1.
Efficient counting: use a suffix automaton or suffix array to count distinct substrings containing at least one '*' after each update.
Time complexity: aim for O(n^2) or better, considering the number of corruptions (up to n) and the cost of updating the data structure.
Edge cases: m larger than total possible distinct substrings containing '*', or permutation array length less than needed.
Space complexity: consider memory usage of the chosen data structure.

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