Decode an Encoded String

Given an encoded string s, decode it according to the following rule and output the full decoded string: An encoding has the form k[encoded_string], meaning that the string inside the brackets is repeated k times. k is a positive integer and may contain multiple digits. Encodings may be nested. The input is guaranteed to be valid. The original strings contain no digits; digits are used only as repetition counts. Implement a decoder. Example 1: Input: 3[a]2[bc] Output: aaabcbc Example 2: Input: 3[a2[c]] Output: accaccacc Example 3: Input: 2[abc]3[cd]ef Output: abcabccdcdcdef Constraints: 1 <= len(s) <= 10^5 Repetition counts are positive integers. The decoded output length is at most 10^6. Follow-up: How would you redesign the solution to reduce extra memory usage if the decoded string can be too large to fit in memory at once? Example Input 3[a]2[bc] Output aaabcbc