Bytedance·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Bytedance software engineer interview with a parsing/evaluation problem that looked straightforward but had a lot of edge cases packed in. The kind of question where you think you're done and then they keep asking 'what about this input'.

Questions Asked (1)

Write a function that validates and evaluates an arithmetic expression string containing only non-negative integers and the operators + and -. The function should reject invalid inputs (leading zeros, adjacent operators, operator at start or end, invalid characters, numbers or result outside 32-bit signed integer range) and return the computed result for valid ones.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I started coding before I fully thought through the validation rules and it bit me.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Clarify the exact validation rules and edge cases, then propose a single-pass tokenizer/parser that validates and evaluates simultaneously. Discuss how to handle 32-bit overflow and leading zeros, and outline a clean implementation with clear error handling.

Pro tip: Mention that you would use 64-bit arithmetic (e.g., long long) for intermediate calculations to detect overflow before it happens, and that you would write unit tests for edge cases like '0', '1+2-3', and '2147483647+1'.

1. Clarify requirements and edge cases

Ask about the exact definition of invalid inputs: leading zeros (e.g., '01'), adjacent operators (e.g., '1++2'), operator at start/end, invalid characters, and 32-bit range for numbers and result. Confirm whether whitespace is allowed.

2. Design a single-pass validation and evaluation strategy

Propose parsing the string left-to-right, maintaining a running result and a sign for the next number. Validate each character and number as you go, rejecting invalid patterns immediately.

3. Handle number parsing and overflow

Parse consecutive digits into a number, checking for leading zeros (unless the number is exactly '0') and ensuring the number fits in a 32-bit signed integer. Use 64-bit arithmetic for intermediate sums to detect result overflow.

4. Implement operator handling and final validation

Apply the operator to the running result, ensuring no two operators are adjacent and that the expression does not start or end with an operator. After processing, verify the result is within 32-bit range.

5. Test and discuss trade-offs

Walk through test cases covering valid and invalid inputs, and discuss time/space complexity (O(n) time, O(1) space). Mention alternative approaches like regex or two-pass validation and their trade-offs.

Key Points to Mention

Single-pass parsing with state machine (expecting number vs. operator) to catch adjacent operators and operator at start/end.
Leading zero detection: a number with more than one digit cannot start with '0'.
32-bit signed integer range: -2^31 to 2^31-1 for both individual numbers and final result.
Use of 64-bit intermediate arithmetic to prevent overflow during addition/subtraction.
Handling of invalid characters: only digits, '+', and '-' are allowed (no spaces unless specified).
Edge cases: empty string, single number, expression with only operators, and maximum/minimum values.

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