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Parse a nested list from a string | TikTok Coding Question

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.

Input: "[10,[20,[30]],40]"

Expected Output: [10, [20, [30]], 40]

Explanation: This checks multiple nesting levels with multi-digit values.

Input: "[ -1, 23, [ 0, -999 ], [] ]"

Expected Output: [-1, 23, [0, -999], []]

Explanation: Whitespace should be ignored, negative values should parse correctly, and empty sublists should be preserved.

Input: "[[],[[]],-5,12345]"

Expected Output: [[], [[]], -5, 12345]

Explanation: This tests empty nested lists, a negative number, and a larger multi-digit number.

Input: " [[[1]], 2] "

Expected Output: [[[1]], 2]

Explanation: Leading and trailing whitespace should not affect parsing.

Hints

Opening and closing brackets suggest using a stack to keep track of the current list being built.
When you encounter a digit or a minus sign, consume the entire integer before continuing the scan.

Quick Overview

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.