Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026Remote

Summary

Meta phone screen for a software engineering role, one coding question the whole time. The problem looked deceptively clean but the follow-up pushed into territory I wasn't fully ready for.

Questions Asked (1)

Given a string, repeatedly remove any maximal contiguous run of identical characters with length at least 2, concatenating the remaining parts each time, until no such run exists. Return the final string. Design a linear-time, linear-space solution, explain why it's correct, and analyze complexity. Follow-up: generalize to removing runs of length at least k for arbitrary k >= 2.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I got the stack approach pretty quickly: track each character alongside a count, and whenever a count hits 2 (or k in the follow-up), collapse it.

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AI HintsAI Generated

Suggested Approach

Use a stack to process the string character by character, merging with the top of the stack when a run of length >= 2 is formed, and then repeatedly removing any newly formed runs at the top. This yields an O(n) time and O(n) space solution. For the follow-up, adapt the stack to track run lengths and remove runs of length >= k.

Pro tip: Emphasize that the stack approach naturally handles cascading removals and is optimal; mention that a naive simulation would be O(n^2) due to repeated scans. For the follow-up, note that the same stack logic works with a threshold k, but careful implementation is needed to avoid missing cascades.

1. Clarify the problem and edge cases

Restate the problem to ensure understanding: repeatedly remove maximal runs of length >= 2 until none exist. Discuss edge cases: empty string, no runs, entire string removed, and overlapping runs after concatenation.

2. Propose a stack-based solution

Explain that a stack can simulate the process in one pass: push characters, and when the top forms a run of length >= 2, pop the entire run. This automatically handles concatenation and cascading removals.

3. Detail the algorithm and prove correctness

Describe the algorithm step-by-step: iterate through the string, push each character onto the stack, and after each push, check if the top run has length >= 2; if so, pop it. Argue correctness by showing the stack maintains the invariant that it represents the reduced string after processing the prefix, and that any removal is applied immediately.

4. Analyze complexity

Each character is pushed and popped at most once, so time is O(n). The stack uses O(n) space in the worst case. This meets the linear-time, linear-space requirement.

5. Generalize to runs of length >= k

Modify the stack to store characters along with their run lengths. When a run reaches length k, pop it. This still runs in O(n) time and O(n) space, as each character is processed once.

Key Points to Mention

Stack-based simulation avoids O(n^2) naive repeated scanning.
Invariant: stack represents the fully reduced string for the processed prefix.
Cascading removals are handled naturally by checking the top after each push/pop.
Time complexity O(n) because each character is pushed and popped at most once.
Space complexity O(n) for the stack.
Generalization to k: store run lengths in the stack and pop when length >= k.

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