Decompress an encoded string

Q: Decompress an encoded string

This question evaluates string parsing and manipulation skills, focusing on handling nested encodings and repetition semantics in serialized formats. Commonly asked in Coding & Algorithms interviews for Data Scientist roles, it tests reasoning about nested structures and robustness of parsing logic, representing a practical application–level problem in the domain of string processing and parsing rather than a purely theoretical exercise.

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Question

Given an encoded string, return its decoded (decompressed) form.

Use the following encoding rule:

A positive integer $k$ followed by a bracketed substring means the substring is repeated $k$ times.
Encodings can be nested.

Examples:

Input: "3[a]2[bc]" → Output: "aaabcbc"
Input: "3[a2[c]]" → Output: "accaccacc"
Input: "2[abc]3[cd]ef" → Output: "abcabccdcdcdef"

Assume:

The input is valid.
Digits only appear as repeat counts.

Return the decoded string.

Decompress an encoded string

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