Decode nested bracket string

Decode a string encoded with the rule k[encoded], meaning the bracketed substring is repeated k times; encodings can be nested. k is a positive integer (possibly multiple digits) and the bracketed content is itself a valid encoded string. For example "3[a2[c]]" decodes to "accaccacc" and "2[abc]3[cd]ef" decodes to "abcabccdcdcdef". The input is always well-formed and contains only lowercase letters, digits, and brackets. Return the decoded string.