Decode an encoded string

Q: Decode an encoded string

This question evaluates string parsing and manipulation skills, particularly handling nested repetition encodings and managing state when expanding bracketed substrings.

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Question

Implement a decoder for an encoded string where repetition is expressed with a number followed by a bracketed substring.

Rules:

A pattern k[substring] means substring repeated k times.
Patterns can be nested.
Characters outside brackets are literal.
k is a positive integer.

Examples:

Input: "3[a]2[bc]" → Output: "aaabcbc"
Input: "3[a2[c]]" → Output: "accaccacc"
Input: "2[ab3[c]]x" → Output: "abcccabcccx"

Return the decoded string.

Decode an encoded string

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