My first instinct was recursion but I pivoted to a stack approach mid-explanation which probably looked a bit shaky.
Use a stack-based approach to handle nested brackets, where each stack frame stores the current string and the repetition count. Iterate through the input, building numbers and strings, and when encountering ']', pop the stack and repeat the enclosed string. Alternatively, use recursion to parse the string, which can be cleaner for nested structures.
Pro tip: Clarify edge cases upfront, such as multi-digit numbers, nested brackets, and empty strings, and discuss time/space complexity. Mention that the stack approach is O(n) time and space, which is optimal.
Restate the problem and ask clarifying questions about input constraints, such as whether k can be multi-digit, if brackets are balanced, and if there are nested encodings. Identify edge cases like empty strings or numbers with multiple digits.
Decide between stack-based iterative parsing or recursive descent. Explain that stack is often preferred for its explicit handling of nesting and avoidance of recursion depth issues.
Describe the step-by-step process: traverse the string, accumulate digits into a number, push current string and number onto stack when encountering '[', and on ']', pop and repeat the string. For recursion, define a helper that parses until a closing bracket.
State that time complexity is O(n) where n is the length of the decoded string (or output size), and space is O(n) for the stack. Walk through an example like '3[a]2[bc]' to verify correctness.
Mention potential optimizations like using a string builder to avoid repeated concatenation, and compare with recursive approach. Highlight trade-offs between iterative and recursive solutions.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.