Decode String

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. Note that `k` is guaranteed to be a positive integer. You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, `k`. For example, there will not be input like `3a` or `2[4]`. The test cases are generated so that the length of the output will never exceed `105`.   **Example 1:** **Input:** s = "3[a]2[bc]" **Output:** "aaabcbc" ``` **Example 2:** **Input:** s = "3[a2[c]]" **Output:** "accaccacc" ``` **Example 3:** **Input:** s = "2[abc]3[cd]ef" **Output:** "abcabccdcdcdef" ```   **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]`.