Decode String

Given an encoded string, return its decoded string where the encoding rule is k[encoded_string] and the encoded_string inside the square brackets is repeated exactly k times. Assume the input string is valid, with well-formed brackets, no extra white spaces, and that digits appear only as repeat numbers.

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].