Decode Nested Repetition Strings with Stacks

Decode strings using the pattern k[encoded], where the bracketed content is repeated k times and may contain nested repetitions. Plain letters remain in order. ### Constraints and Clarifications The source identifies problem394. The [official Decode String reference](https://leetcode.com/problems/decode-string/description/) supplies this practice contract: valid syntax, lowercase letters, positive repeat counts from1 to300, encoded length1–30, and decoded length at most100000. Digits appear only in repeat counts, not as literal output characters. These are reference-context constraints, not additional recalled interview requirements. ### Example "2[a3[b]]c" decodes to "abbbabbbc". ### What a Strong Answer Covers Track multi-digit counts and nested contexts with stacks or equivalent frames. Explain what is saved at an opening bracket, what is restored at a closing bracket, and the cost of materializing expanded output. ### Follow-up Questions - Why does a single repeat counter fail for nested brackets? - How can repeated string concatenation affect complexity?