Given an encoded string, return its decoded version. The encoding rule follows the pattern: $k[encoded~string]$, where the $encoded~string$ is repeated exactly $k$ times.

Note: The $k$ is guaranteed to be a positive integer.

Assume that the input string is always valid, meaning there are no extra spaces, and the square brackets are properly balanced and well-formed. Additionally, assume that the original data contains no digits and that digits are only used for repeating the string.

For example, the input will not contain patterns like $3a$ or $2[4]$, whereas a valid pattern is $2[aj]3$ ...