Problem: Decode String

Statement

Given an encoded string s, return its decoded string.

The encoding rule follows the format k[encoded_string], where the encoded_string enclosed within the square brackets is repeated exactly k times. The value k is guaranteed to be a positive integer.

You may assume that the input string is always valid: there are no extra whitespace characters, and all square brackets are well formed. Additionally, the original data contains no digit characters; digits appear only as repeat counts (k). For instance, inputs such as 3a or 2[4] will not occur.

The test cases are constructed such that the length of the decoded output never exceeds $10^5$.

Note: Nested encodings are possible, such as k1[string1 k2[string2]], where inner patterns are decoded first before the outer repetition is applied.

Constraints:

• $1 \leq$ s.length $\leq 30$

• s consists of lowercase English letters, digits, and square brackets '[' and ']'

• s is guaranteed to be a valid input

• All integers in s are in the range $[1, 300]$ ...