Evaluate Reverse Polish Notation (Stack)
Evaluate the value of an arithmetic expression in Reverse Polish Notation (RPN). Operators include +, -, *, /. Use a Stack to process the tokens.
Why Interviewers Ask This
Spotify engineers frequently ask this to assess your mastery of stack data structures and your ability to handle edge cases in parsing logic. It evaluates whether you can translate a mathematical concept into an efficient algorithm without relying on built-in recursion or complex parsers. They specifically look for candidates who understand time complexity O(n) and space constraints while processing token streams.
How to Answer This Question
1. Clarify the input format and operator rules immediately, noting that RPN requires operands before operators. 2. Propose a linear scan approach using a single stack, explaining why a queue or hash map is unnecessary here. 3. Walk through a concrete example like ['2', '1', '+', '3', '*'] step-by-step on a whiteboard, pushing numbers and popping for operations. 4. Explicitly discuss handling division as integer truncation towards zero, a common pitfall in Java and C++. 5. Conclude by analyzing time complexity as O(n) since each token is processed once, and space complexity as O(n) for the stack in worst-case scenarios.
Key Points to Cover
- Demonstrating clear understanding of operand order during pop operations for non-commutative operators
- Explicitly stating O(n) time complexity and justifying it with the single-pass iteration
- Handling integer division behavior correctly according to language specifications
- Using a stack to avoid recursion overhead and potential stack overflow issues
- Validating assumptions about input validity and error handling strategies
Sample Answer
To evaluate Reverse Polish Notation, I would use a standard stack-based approach because it naturally handles the last-in-first-out nature of nested operations. First, I initialize an empty stack to store integers. Then,…
Common Mistakes to Avoid
- Swapping operands during subtraction or division, leading to incorrect results due to stack LIFO order
- Ignoring integer division truncation rules which differ between languages like Python and Java
- Failing to consider edge cases such as negative numbers or single-element inputs
- Overcomplicating the solution by trying to build a full parser instead of using a simple stack
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