Parse a nested list from a string
Company: TikTok
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Technical Screen
Given a string that encodes a nested list (ArrayList-like notation), parse it into an in-memory nested list structure.
## Input format
- The string contains:
- `[` and `]` to denote lists
- commas `,` as separators
- optional whitespace
- **integers** (possibly negative)
- Examples:
- `"[1,2,[3,-4],5]"`
- `"[]"`
- `"[10,[20,[30]],40]"`
## Output
Return a nested list data structure where each element is either:
- an integer, or
- another nested list
(You may describe the structure as a `NestedElement` type with two variants: `Integer(value)` or `List(children)`.)
## Constraints
- Input is guaranteed to be valid.
- Length of the string can be up to ~10^5 characters, so the solution should be near O(n) time.
## Follow-ups
- How would you handle very deep nesting (stack depth concerns)?
- What edge cases must be tested (empty list, negative numbers, multi-digit values)?
Quick Answer: This question evaluates proficiency in parsing nested data structures, string processing, hierarchical representation construction, and analysis of algorithmic time and space complexity within the Coding & Algorithms domain.
Given a string `s` that encodes a nested list using bracket notation, parse it into an in-memory nested structure. The outermost value is always a list.
The string may contain:
- `[` and `]` to denote lists
- commas `,` as separators
- optional whitespace
- integers, including negative and multi-digit values
Return the parsed structure as nested Python lists, where each element is either:
- an integer, or
- another list
For example:
- `"[1,2,[3,-4],5]"` -> `[1, 2, [3, -4], 5]`
- `"[]"` -> `[]`
- `"[10,[20,[30]],40]"` -> `[10, [20, [30]], 40]`
Do not use `eval`, `json`, or other built-in parsers. Parse the string manually.
Follow-up: how would you handle very deep nesting without risking recursion depth errors?
Constraints
- The input string encodes exactly one valid outer list.
- 2 <= len(s) <= 10^5
- Whitespace may appear anywhere outside of numbers.
- Integers may be negative and may contain multiple digits.
Examples
Input: "[1,2,[3,-4],5]"
Expected Output: [1, 2, [3, -4], 5]
Explanation: The string contains integers at the top level and one nested list.
Input: "[]"
Expected Output: []
Explanation: An empty list should parse to an empty Python list.