Given an encoded string, return its decoded string.
The encoding rule is: `k[encoded_string]`, where the `encoded_string` inside the square brackets is being repeated exactly `k` times.
checklist Constraints
1 <= s.length <= 30 s consists of lowercase English letters, digits, and square brackets '[]'. s is guaranteed to be a valid input. All the integers in s are in the range [1, 300].