Decode String

LeetCode 394. Decode `k[group]` expansion. Iterative stack solution; Google variants swap the syntax (e.g. `(group){k}`) or add nesting rules. Requirements Input: encoded string like "3[a2[c]]". Output: decoded expansion "accaccacc". Iterative stack: keep a stack of (prev_string, repeat_count); on ], pop and concatenate. Examples "3[a]2[bc]" → "aaabcbc". "2[abc]3[cd]ef" → "abcabccdcdcdef". "3[a2[c]]" → "accaccacc". Notes Google variants include alternative syntax (group){k} (postfix repeat), nested grouping with curly braces, and escape characters. A scale follow-up asks how the interface and memory strategy should change when the encoded string is extremely long. Recursive solution is cleaner but the iterative stack is more robust to bizarre input. Preparation Write the iterative-stack version from memory in under 10 min. Practice the postfix-syntax variant (parse direction reversed). Drill the decode side together with the encode-shortest dual problem as a pair.