LinkedIn·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

LinkedIn software engineer interview, coding round. Pretty standard bracket matching problem but they layered on a follow-up that tripped me up more than I expected.

Questions Asked (1)

Given a string of bracket characters, write a function to determine if the brackets are valid (every opening bracket has a matching closing bracket in the correct order). Start with only parentheses, then extend to all three bracket types.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Part A felt easy and I went straight to a counter, just incrementing on '(' and decrementing on ')'.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then propose a stack-based solution for parentheses. Walk through the algorithm step-by-step, and extend it to handle all three bracket types by using a mapping of closing to opening brackets. Analyze time and space complexity, and discuss potential optimizations or alternative approaches.

Pro tip: Mention that you would use a stack and a hash map for matching brackets, and emphasize that this approach naturally handles nested and interleaved brackets. Also, proactively discuss edge cases like empty strings and strings with only closing brackets.

1. Clarify requirements and edge cases

Ask if the string can be empty, if it contains only brackets, and if we need to handle all three types. Confirm that validity means every opening bracket is closed by the same type in the correct order.

2. Propose stack-based approach

Explain that a stack is ideal because brackets must be closed in LIFO order. For parentheses only, push opening brackets and pop when a closing bracket is encountered, checking for emptiness.

3. Extend to multiple bracket types

Introduce a mapping (e.g., hash map) from closing to opening brackets. When a closing bracket is seen, check if the stack is non-empty and the top matches the expected opening bracket.

4. Walk through an example

Trace the algorithm on a sample string like '{[()]}' to demonstrate correctness, and also on an invalid string like '([)]' to show how mismatches are caught.

5. Analyze complexity and discuss trade-offs

State that time complexity is O(n) and space complexity is O(n) in the worst case. Mention that this is optimal for a single-pass solution, and briefly discuss alternatives like using a counter for parentheses only (but not for multiple types).

Key Points to Mention

Use a stack to track opening brackets.
Use a hash map to match closing brackets to their corresponding opening brackets.
Check for empty stack when encountering a closing bracket.
After processing all characters, ensure the stack is empty.
Time complexity: O(n), space complexity: O(n).
Edge cases: empty string, string with only closing brackets, mismatched types.

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