I went straight for a stack-based approach and it mostly worked, but I fumbled on nested brackets for longer than I should have.
Use a stack to handle nested encoded strings, pushing current string and repeat count when encountering '[', and popping and expanding when encountering ']'. Alternatively, use recursion to decode each bracket level. Clearly explain the algorithm, then analyze time and space complexity.
Pro tip: Mention that the time complexity is O(n) where n is the length of the decoded string, and space is O(n) for the stack; also note that recursion depth could be an issue for deeply nested strings, so an iterative stack approach is often preferred in production code.
Restate the problem to ensure understanding, and ask about edge cases like empty strings, multiple digits for repeat counts, and nested brackets. Confirm that the input is always valid.
Decide between using a stack or recursion. Explain that a stack is iterative and avoids recursion depth limits, while recursion is more intuitive for nested structures.
Describe step-by-step how to parse the string: maintain a current string and repeat count, push them onto the stack when encountering '[', and on ']' pop and repeat the current string, then append to the previous string.
State that time complexity is O(n) where n is the length of the decoded string, and space is O(n) for the stack. Discuss handling of multi-digit numbers and nested brackets.
Walk through a simple example like '3[a]2[bc]' and a nested one like '3[a2[c]]' to demonstrate correctness. Mention potential pitfalls like integer overflow for large repeat counts.
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