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Jane Street·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Jane Street SWE interview with a two-part coding problem centered on building a collapsible code editor feature. The problem was more involved than it looked at first glance, especially the nesting independence requirement in part two.

Questions Asked (2)

Q1

Given a list of lines representing source code where each line has at most one brace as its last character, write a function that returns a mapping from each opening brace's line index to its matching closing brace's line index, in a single pass.

Algorithms & Data Structures
Author's notes

Stack-based solution, pretty standard once you see it.

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AI HintsAI Generated

Suggested Approach

Use a stack to track the indices of opening braces as you iterate through the lines. When you encounter a closing brace, pop the top of the stack and record the mapping from the popped index to the current line index. This achieves a single pass with O(n) time and O(n) space.

Pro tip: Clarify assumptions upfront: whether braces are guaranteed to be balanced, whether lines can have braces in strings/comments, and whether the input is a list of strings or a single string. Handling edge cases like unbalanced braces gracefully shows attention to detail.

1. Clarify requirements and edge cases

Ask about input format, brace balance guarantees, and whether braces can appear in comments or strings. Confirm that each line has at most one brace as its last character.

2. Choose data structure

Select a stack to store indices of opening braces. This naturally handles nesting and ensures O(1) push/pop operations.

3. Iterate through lines

For each line index i, check the last character. If it's an opening brace, push i onto the stack. If it's a closing brace, pop from the stack and map the popped index to i.

4. Handle edge cases

If a closing brace appears with an empty stack, it's unbalanced; decide whether to ignore, raise an error, or handle as per requirements. After iteration, if stack is non-empty, there are unmatched opening braces.

5. Return the mapping

Return the dictionary mapping opening brace indices to closing brace indices. Discuss time and space complexity: O(n) time, O(n) space in worst case.

Key Points to Mention

  • Stack data structure for matching nested braces
  • Single pass O(n) time complexity
  • Space complexity O(n) due to stack and output map
  • Handling of unbalanced braces (e.g., closing without opening, opening without closing)
  • Assumption that braces are the last character on a line
  • Potential need to ignore braces in strings/comments (if applicable)

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

Q2

Extend the brace-matching solution with a toggle and getText interface that supports collapsing and expanding individual brace pairs, where each pair's collapsed state is fully independent regardless of nesting.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

The independence requirement is what makes this genuinely tricky.

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AI HintsAI Generated

Suggested Approach

First, clarify the requirements and constraints, especially what 'fully independent' means for nested pairs. Then, propose a data structure that maps each brace pair to its own collapsed state, and design the toggle and getText methods to respect those states. Finally, discuss trade-offs and edge cases, such as performance and handling of malformed input.

Pro tip: Emphasize that independence means collapsing an outer pair should not affect inner pairs, and vice versa; this often requires storing state per pair rather than per nesting level. Also, mention that getText should reconstruct the string efficiently, possibly using a stack or tree traversal.

1. Clarify Requirements

Ask questions to confirm what 'collapsed' means (e.g., replace with ellipsis or remove entirely) and how independence should behave for nested pairs. Confirm the interface: toggle(pairId) and getText().

2. Design Data Structure

Propose a structure to identify each brace pair uniquely (e.g., by start index) and store its collapsed state. Consider using a map from pair identifier to boolean, or augmenting a parse tree with state.

3. Implement toggle and getText

Describe how toggle flips the state for a given pair. For getText, outline an algorithm that traverses the original string or parse tree, skipping or replacing content of collapsed pairs while preserving others.

4. Analyze Complexity and Trade-offs

Discuss time and space complexity of toggle and getText. Compare approaches: precomputing a tree vs. on-the-fly parsing. Mention how independence affects the algorithm.

5. Handle Edge Cases

Address malformed input, overlapping pairs, and performance for large inputs. Suggest optimizations like lazy evaluation or caching.

Key Points to Mention

  • Unique identification of brace pairs (e.g., by start index or object reference).
  • Storing collapsed state independently per pair, not per nesting level.
  • Efficient getText implementation, possibly using a stack or tree traversal.
  • Handling nested pairs: collapsing outer does not collapse inner, and vice versa.
  • Time and space complexity of toggle and getText operations.
  • Edge cases: malformed input, empty pairs, and performance for large strings.

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