Palo Alto Networks·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Palo Alto Networks coding round for a Software Engineer position. One algorithmic question that seemed manageable at first but had a follow-up that pushed things further than I expected.

Questions Asked (1)

Given a string representing a basic arithmetic expression with non-negative integers and the operators +, -, *, /, evaluate it and return the result. Integer division should truncate toward zero. You cannot use any built-in eval-style functions. Follow-up: extend the solution to handle parentheses.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Got the base case working with a stack, tracking sign and handling operator precedence by pushing/popping for * and /.

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AI HintsAI Generated

Suggested Approach

Start by clarifying constraints (e.g., spaces, operator precedence, integer division truncation) and then propose a two-pass or stack-based solution for the basic expression. For the follow-up, extend the same approach by using a stack to handle parentheses, or switch to a recursive descent parser. Emphasize clean code, edge cases, and time/space complexity.

Pro tip: Mention that you would write unit tests for edge cases like division by zero, negative intermediate results, and nested parentheses, and discuss how you'd handle operator precedence without eval. This shows production-level thinking.

1. Clarify requirements and constraints

Ask about input format (spaces, unary operators, division by zero), expected output type, and whether parentheses are part of the initial problem or only the follow-up. Confirm integer division truncates toward zero.

2. Outline a stack-based approach for basic expression

Explain that you'll parse the string, maintain a stack for numbers, and handle * and / immediately (higher precedence) while deferring + and - by pushing signed numbers. Then sum the stack.

3. Extend to parentheses with recursion or stack

For parentheses, describe either a recursive descent parser that evaluates subexpressions when encountering '(' or a stack-based method that pushes the current result and sign onto a stack when seeing '(' and restores them at ')'.

4. Analyze complexity and trade-offs

State that both approaches run in O(n) time and O(n) space. Discuss trade-offs: recursive descent is more readable but uses call stack; stack-based is iterative but may be trickier to implement.

5. Discuss edge cases and testing

Mention handling of division by zero (throw error or return sentinel), negative numbers, multiple-digit numbers, and nested parentheses. Suggest writing tests for these cases.

Key Points to Mention

Operator precedence: * and / before + and -
Integer division truncation toward zero (e.g., -3/2 = -1)
Stack-based evaluation for basic expression
Recursive descent or stack-based approach for parentheses
Time and space complexity: O(n) time, O(n) space
Edge cases: division by zero, negative numbers, nested parentheses, whitespace

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