Decode a string encoded in the format "k[encoded_string]" where the encoded_string inside the brackets is repeated k times. The encoding can be nested. Return null for null input and empty string for empty input.
Rules:
- k is a positive integer
- encoded_string contains only letters and may include other encoded patterns
- Nesting is supported: "3[a2[c]]" decodes to "accaccacc"
Examples:
Input: encoded = '3[a]2[bc]'
Output: 'aaabcbc'
Explanation: '3[a]' decodes to 'aaa' and '2[bc]' decodes to 'bcbc'
Input: encoded = '3[a2[c]]'
Output: 'accaccacc'
Explanation: Inner '2[c]' decodes to 'cc', making 'acc', then '3[acc]' gives 'accaccacc'
Input: encoded = '2[abc]3[cd]ef'
Output: 'abcabccdcdcdef'
Explanation: '2[abc]' gives 'abcabc', '3[cd]' gives 'cdcdcd', then 'ef' is appended
Input: encoded = 'abc'
Output: 'abc'
Explanation: No brackets, return as is
Input: encoded = null
Output: null
Explanation: Null input returns null
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