Parse a nested list from a string

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)?