MathWorks·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

MathWorks software engineer interview with a pretty gnarly string transformation problem. The algorithmic depth here was real, not your typical LeetCode warm-up.

Questions Asked (1)

Given two equal-length strings s and t over lowercase letters, you can repeatedly pick any character and replace all its occurrences in s with another character. The mapping applies globally and persists. Determine if s can be transformed into t, handling cycles by using a spare character as a temporary buffer if one exists outside t's character set. Design an algorithm and output a valid replacement sequence if possible.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This one took me a while to even parse correctly.

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AI HintsAI Generated

Suggested Approach

Model the transformation as a directed graph where each character in s maps to the corresponding character in t. Check for consistency (no character maps to two different targets) and handle cycles by using a spare character not present in t as a temporary buffer. Then produce a valid sequence of replacements by processing mappings in topological order, breaking cycles with the spare character if available.

Pro tip: Clarify edge cases upfront: if s equals t, no operations are needed; if a cycle exists and no spare character is available, transformation is impossible. Also, mention that the spare character must not appear in t to avoid unintended mappings.

1. Validate mapping consistency

Iterate through both strings and build a mapping from each character in s to its corresponding character in t. If any character in s maps to two different characters, return impossible.

2. Identify cycles and spare character

Detect cycles in the mapping graph. Determine if there is a spare character (a lowercase letter not present in t) that can be used to break cycles.

3. Handle cycles with spare character

If cycles exist and a spare character is available, break each cycle by temporarily mapping one character to the spare, then completing the cycle, and finally mapping the spare to the correct target.

4. Generate replacement sequence

Process mappings in an order that respects dependencies (e.g., topological order for acyclic parts). For cycles, output the sequence of replacements including the temporary spare character steps.

5. Verify and output

Simulate the sequence on s to ensure it transforms to t, then output the sequence or indicate impossibility.

Key Points to Mention

Graph representation of character mappings and detection of conflicts.
Cycle detection in the mapping graph (e.g., using DFS or union-find).
Role of a spare character not in t for breaking cycles.
Topological ordering for acyclic mappings to avoid overwriting.
Edge cases: s already equals t, no spare character available, multiple cycles.
Time and space complexity: O(n) for validation and O(1) for character set operations.

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