Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Meta SWE interview with a bracket-removal problem. The core challenge was figuring out the minimum deletions to make a string's brackets valid and properly nested, which sounds straightforward until you actually try to implement it cleanly.

Questions Asked (1)

Given a string containing letters and bracket characters from the set (), [], {}, remove the minimum number of brackets so that the remaining brackets are properly matched and nested. Non-bracket characters must stay in their original positions. Return any valid result.

Algorithms & Data Structures

Author's notes

The example they gave was something like `([{ab)` and the answer was `(ab)`.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Use a stack to identify unmatched brackets in a single pass, then remove those unmatched brackets to produce a valid string. Alternatively, mark matched brackets and build the result from the original string, preserving non-bracket characters.

Pro tip: Clarify that 'minimum removal' equals removing exactly the unmatched brackets, and mention that multiple valid answers exist; focus on correctness and efficiency.

1. Clarify requirements and edge cases

Confirm that non-bracket characters must remain in place, and that any valid result is acceptable. Discuss edge cases like empty string, all brackets, or already balanced.

2. Choose an approach

Decide between stack-based marking of unmatched brackets or using a stack to build the result directly. Both achieve O(n) time and space.

3. Implement the algorithm

Traverse the string, using a stack to track opening brackets and their indices. Mark closing brackets that match, and at the end, mark any remaining opening brackets as unmatched.

4. Construct the result

Build a new string by including only characters that are not marked for removal, preserving the original order of non-bracket characters.

5. Test and verify

Walk through examples to ensure the result is balanced and non-bracket characters are in their original positions. Check time and space complexity.

Key Points to Mention

Use a stack to track opening brackets and their indices for efficient matching.
Mark unmatched brackets (both opening and closing) for removal.
Preserve non-bracket characters in their original positions.
Time complexity O(n) and space complexity O(n) due to stack and marking array.
Multiple valid answers may exist; any correct result is acceptable.
Handle edge cases such as empty string, no brackets, or already balanced string.

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