Seen this one floating around in other posts so I wasn't totally blindsided.
Start by clarifying the problem scope: input format, expected output, and constraints. Then outline a simple algorithm using a stack or two-pass approach to handle operator precedence, and discuss edge cases like negative numbers and whitespace. Finally, walk through a concrete example to validate the solution.
Pro tip: Mention that you would write unit tests for edge cases like '1+2-3' and ' 3 - 2 ' to ensure robustness, and discuss how to extend the solution to support multiplication and division if needed.
Ask about input format (string, tokens, etc.), output type, and constraints (e.g., integer overflow, whitespace handling). Confirm if only addition and subtraction are required.
Decide between a stack-based evaluation or a two-pass method (first handle addition/subtraction, then combine). Explain why the chosen approach is efficient and easy to implement.
Identify and address edge cases such as negative numbers, multiple operators in a row, leading/trailing spaces, and empty input.
Write pseudocode or actual code, then walk through a few test cases to verify correctness. Discuss time and space complexity.
Mention how the solution could be extended to support multiplication, division, parentheses, or floating-point numbers, showing awareness of scalability.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
The follow-up is where it gets a bit more interesting.
First, clarify the requirements: what constitutes a valid expression (allowed operators, parentheses, numbers, whitespace, etc.) and what should happen on invalid input. Then, outline a validation strategy, such as tokenizing and using a state machine or recursive descent parser, and discuss trade-offs between strict and lenient validation. Finally, explain how you would integrate validation into the existing calculator without duplicating logic.
Pro tip: Mention that validation should be a separate, testable component that returns meaningful error messages, and that it can be reused for both client-side and server-side validation to ensure consistency and security.
Ask the interviewer about the expected input format, allowed characters, and behavior on invalid input (e.g., throw exception, return error). Identify edge cases like empty string, unbalanced parentheses, consecutive operators, and division by zero.
Decide between a simple regex, a state machine, or a full parser. Discuss trade-offs: regex is quick but limited for nested structures; a state machine is more robust; a parser can validate and evaluate simultaneously.
Outline the steps: tokenize the input, then iterate through tokens while maintaining state (e.g., expecting operand or operator). Handle parentheses by tracking depth. Return a boolean or throw an error with details.
Modify the calculator to call the validator before evaluation. Ensure that validation errors are propagated clearly. Avoid duplicating parsing logic by possibly reusing the tokenizer.
Propose test cases for valid and invalid expressions. Discuss performance implications (e.g., O(n) time) and whether validation should be strict or allow some flexibility (e.g., implicit multiplication).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.