Meta·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

Meta coding screen, one question the whole time. Pretty focused on expression parsing which I don't practice enough. Left feeling like I got through it but not cleanly.

Questions Asked (1)

Write a function that evaluates a math expression given as a string. The expression contains non-negative integers, plus, minus, multiply, and divide operators, and spaces. No parentheses. Division truncates toward zero. No built-in eval allowed. Must handle long inputs efficiently, and you need to explain time and space complexity.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just scan left to right and apply ops as I go, which breaks immediately once you account for operator precedence.

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AI HintsAI Generated

Suggested Approach

Use a single-pass stack-based algorithm: parse the string into tokens, then process multiplication and division immediately while pushing addition and subtraction results onto a stack. Finally, sum the stack to get the result. This handles operator precedence without parentheses and runs in O(n) time.

Pro tip: Mention that you can avoid a separate tokenization pass by parsing numbers and operators on the fly, and explicitly discuss how you handle division truncation toward zero for negative intermediate results (e.g., using truncation instead of floor).

1. Clarify requirements and edge cases

Confirm input constraints: non-negative integers, operators + - * /, spaces, no parentheses, division truncates toward zero. Discuss handling of large numbers, empty strings, and potential overflow.

2. Choose the algorithm

Select a stack-based approach to handle operator precedence: process * and / immediately, and push + and - terms onto a stack. Alternatively, use two stacks (operands and operators) but the single stack is simpler.

3. Implement parsing and evaluation

Iterate through the string, building numbers and tracking the last operator. When an operator is encountered, apply the previous operator to the current number and the top of the stack (for * and /) or push the number (for + and -). Handle spaces by skipping them.

4. Handle division truncation

Ensure division truncates toward zero. In languages like Python, use int(a / b) or math.trunc(a / b) instead of // (which floors). In Java/C++, integer division already truncates toward zero.

5. Analyze complexity and test

State time complexity O(n) and space complexity O(n) in the worst case (e.g., all additions). Walk through examples like '3+2*2', ' 3/2 ', and '3+5 / 2' to verify correctness.

Key Points to Mention

Operator precedence: multiplication and division must be evaluated before addition and subtraction.
Stack usage: push numbers for addition/subtraction, but for multiplication/division, pop the last number, compute, and push the result.
Division truncation toward zero: use truncation, not floor, especially for negative intermediate results.
Time complexity O(n) and space complexity O(n) due to stack; explain why it's optimal for this problem.
Handling spaces: skip them during parsing, and ensure number parsing works with multi-digit numbers.
Edge cases: single number, leading/trailing spaces, division by zero (though not specified, mention it).

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