Jump Trading·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Jump Trading SWE interview with a stack-based string processing problem. Pretty algorithmic, felt like a coding screen more than anything else.

Questions Asked (1)

Given a string of whitespace-separated tokens (non-negative integers, '+', '-', 'DUP', 'POP'), simulate a stack machine: push integers, apply arithmetic ops on the top two elements, duplicate or pop the top, return an error on any illegal state, and return the top of the stack after all tokens are processed.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The operand order for '+' and '-' tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Clarify the exact semantics of each operation and error conditions, then outline a stack-based simulation with careful edge-case handling. Discuss time/space complexity and potential optimizations or trade-offs before coding.

Pro tip: Explicitly define what constitutes an 'illegal state' (e.g., insufficient operands, division by zero, stack underflow) and confirm with the interviewer; this shows attention to detail and prevents ambiguity.

1. Clarify Requirements and Edge Cases

Ask about token types, error handling (e.g., return error message or exception), integer overflow, and whether division is integer or float. Confirm stack behavior for each operation.

2. Design the Algorithm

Use a stack (list) to store integers. Iterate through tokens: push numbers, apply ops on top two elements (checking for at least two), DUP duplicates top (check non-empty), POP removes top (check non-empty).

3. Handle Errors and Edge Cases

For each operation, validate preconditions (e.g., stack size >= 2 for arithmetic, >= 1 for DUP/POP). Return an error immediately if violated. Also handle empty stack at end (return error or 0?).

4. Analyze Complexity and Trade-offs

Time: O(n) for n tokens, space: O(n) worst-case. Discuss if a more memory-efficient approach exists or if error handling could be optimized.

5. Test with Examples

Walk through a sample input like '3 4 + DUP *' to verify correctness, and test error cases like '1 +' or 'POP' on empty stack.

Key Points to Mention

Stack data structure and LIFO principle
Error conditions: stack underflow, insufficient operands, division by zero
Time and space complexity analysis (O(n) time, O(n) space)
Integer overflow considerations and whether to use arbitrary precision
Order of operands for subtraction and division (e.g., second popped minus first popped)
Handling of empty stack at the end (return error or 0?)

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