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Snowflake·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Snowflake software engineer interview with a classic parentheses balancing problem that escalated pretty fast into multi-bracket territory. Not a bad experience but the follow-up caught me more off guard than it should have.

Questions Asked (1)

Q1

Given a string of only '()' characters, determine whether the parentheses are balanced. Then extend your solution to handle '[]' and '{}' with correct nesting and ordering. Walk through your algorithm, the data structure you'd use, time and space complexity, and edge cases like empty strings or unexpected characters.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Started fine with the basic version, stack-based, push on open and pop and check on close.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and constraints, then propose a stack-based solution for the extended case, explaining how it naturally handles the simple case. Walk through the algorithm step-by-step, analyze time and space complexity, and discuss edge cases and potential optimizations.

Pro tip: Mention that you can early-exit if the string length is odd, and that using a stack with a hash map for matching brackets makes the code clean and extensible. Also, discuss how this approach can be adapted for streaming input if needed.

1. Clarify requirements and constraints

Ask about input size, character set, and whether the string can be empty or contain unexpected characters. Confirm that the goal is to check for balanced and correctly nested brackets.

2. Propose a stack-based solution

Explain that a stack is ideal because brackets must be closed in reverse order of opening. For the simple case, you can just count, but for multiple types, a stack is necessary.

3. Walk through the algorithm

Iterate through each character: if it's an opening bracket, push onto the stack; if it's a closing bracket, check if the stack is empty or if the top doesn't match, then return false; otherwise pop. At the end, return true only if the stack is empty.

4. Analyze complexity and edge cases

Time complexity is O(n) since each character is processed once. Space complexity is O(n) in the worst case (e.g., all opening brackets). Discuss edge cases: empty string (balanced), odd length (immediately unbalanced), unexpected characters (either ignore or return false based on requirements).

5. Discuss optimizations and trade-offs

Mention early termination for odd lengths, using a hash map for matching pairs, and potential memory optimizations if only one type of bracket is used. Also, consider if the input is a stream and how to handle it.

Key Points to Mention

  • Use a stack to track opening brackets and ensure correct nesting and ordering.
  • Time complexity O(n) and space complexity O(n) due to stack usage.
  • Edge cases: empty string, odd length, unexpected characters, and strings with only closing brackets.
  • Early exit if string length is odd, as it cannot be balanced.
  • Use a hash map to map closing brackets to their corresponding opening brackets for clean code.
  • Discuss potential follow-ups: handling multiple types, streaming input, or memory constraints.

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