Classic stack problem once you see it, but I fumbled around for a minute trying to think of a simulation approach before the stack click happened.
Use a stack to process the string character by character: if the current character matches the top of the stack, pop it; otherwise, push it. This naturally handles cascading removals in a single pass, yielding O(n) time and O(n) space.
Pro tip: After presenting the stack solution, mention that a two-pointer in-place approach can achieve O(1) extra space if the input is a mutable array, showing you optimize for memory when appropriate.
Ask about input constraints (e.g., character set, string length, mutability) and confirm that removal is case-sensitive and applies repeatedly until no adjacent duplicates remain.
Describe iterating through the string and using a stack: push if different from top, pop if same. This simulates the repeated removal process in one pass.
Trace the algorithm on a sample like 'abbaca' to demonstrate how cascading removals happen (e.g., 'bb' removed, then 'aa' removed, leaving 'ca').
State that time complexity is O(n) because each character is pushed and popped at most once, and space complexity is O(n) for the stack in the worst case.
Mention the two-pointer in-place variant for O(1) extra space, and handle edge cases like empty string, all duplicates, and no duplicates.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.