I got the basic structure pretty fast, like yes, stack, okay.
Use a stack to handle nested repetitions: push the current string and repeat count when encountering '[', and pop and expand when encountering ']'. Alternatively, use recursion to parse the string, treating each bracket pair as a recursive call. Both approaches run in O(n) time and O(n) space.
Pro tip: Clarify edge cases upfront (e.g., multi-digit numbers, empty brackets, invalid input) and discuss how your solution handles them. Mention that you can optimize space by using a single stack storing both counts and strings, or by using recursion with an index pointer.
Restate the problem to ensure clarity, ask about input size, character set, and whether the input is always valid. Discuss potential edge cases like multi-digit numbers, nested brackets, and empty strings.
Decide between stack-based iterative and recursive solutions. Explain the trade-offs: stack is explicit and avoids recursion depth issues; recursion is cleaner but may hit stack limits for deep nesting.
For stack: iterate through the string, building the current string and number. On '[', push current string and number onto stacks, reset them. On ']', pop and repeat the current string, then append to the previous string. For recursion: parse until ']', recursively decode inner parts.
State time and space complexity: O(n) time where n is the length of the decoded string (or input length, depending on analysis), and O(n) space for the stack. Discuss handling of multi-digit numbers and nested brackets.
Walk through the given example '3[a2[c]]' step by step, showing how the stack evolves. Also test edge cases like '2[abc]3[cd]ef', '10[a]', and '3[a2[c]b]'.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.