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Pretty approachable once you realize it's just two passes with a hash set.
Use two passes with a hash set: first pass left-to-right to mark if each element has been seen before, second pass right-to-left to mark if each element will be seen again. Build the two binary strings accordingly, ensuring O(n) time and O(n) space.
Pro tip: Clarify the definition of 'appeared before' and 'appears again'—whether it's based on value or index—and confirm the output format (e.g., '1' for true, '0' for false). Also, mention edge cases like empty array or single element.
Restate the problem in your own words and ask clarifying questions about the definition of 'appeared before' and 'appears again' (value-based vs. index-based) and the expected output format.
Select a hash set to track seen elements for O(1) lookups. Use two passes: one left-to-right for the 'before' string, one right-to-left for the 'after' string.
In the first pass, for each element, check if it's in the set; if yes, append '1' to the before string, else '0', then add the element to the set. In the second pass, do the same from right to left for the after string, then reverse it.
Test with empty array, single element, all duplicates, and all unique elements. Ensure the strings are of correct length and characters are '0' or '1'.
State that time complexity is O(n) and space complexity is O(n) due to the hash set and output strings. Discuss potential optimizations if needed.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
First, clarify the problem and edge cases, then derive an efficient solution by observing that each '1' moves right past '0's, and the stabilization time equals the maximum number of '0's any '1' must cross. Use a single pass to compute this maximum, explaining the reasoning clearly.
Pro tip: Mention that a naive simulation is O(n^2) and would fail for large inputs, then present the O(n) insight—this shows you think about scalability and optimization, which interviewers value.
Confirm that replacements happen simultaneously each second, and that the string stabilizes when no '01' remains. Ask about input size and constraints.
Walk through small examples (e.g., '01', '0011', '0101') to observe the pattern and verify your understanding of the process.
Recognize that each '1' moves right past '0's, and the total time is the maximum number of '0's that any '1' must cross. This can be computed by scanning left to right, counting zeros and ones.
Use a single pass: maintain a count of zeros seen so far and the maximum steps for any '1'. When encountering a '1', update the maximum with the current zero count; when encountering a '0', increment the zero count.
State that the algorithm runs in O(n) time and O(1) space. Test edge cases like all zeros, all ones, and alternating patterns.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.