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Sort by start, then walk through and merge if the current interval overlaps the last one in your result list.
Start by clarifying edge cases like empty input, single interval, and whether intervals are inclusive/exclusive. Then propose sorting intervals by start time and merging in a single pass, explaining the O(n log n) time and O(n) space complexity. Finally, walk through a concrete example to demonstrate correctness and discuss potential optimizations or variations.
Pro tip: Mention that sorting is the key insight because it reduces the problem to a linear scan, and proactively discuss how you'd handle edge cases like adjacent intervals (e.g., [1,2] and [2,3]) depending on whether they should merge.
Ask about input constraints, interval inclusivity, and expected output format. Confirm handling of empty lists, single intervals, and unsorted input.
Explain that you will sort intervals by start time, then iterate and merge overlapping intervals by comparing the current end with the next start.
State that sorting dominates at O(n log n) time, and the merge pass is O(n), resulting in O(n log n) overall. Space is O(n) for the output (or O(1) extra if merging in-place).
Use a small example like [[1,3],[2,6],[8,10],[15,18]] to show how the algorithm merges [1,3] and [2,6] into [1,6], and produces the final sorted list.
Cover cases like intervals that touch at endpoints, large inputs, and whether to merge adjacent intervals. Mention possible optimizations or alternative approaches if asked.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Clarify the problem by walking through a small example to confirm understanding, then propose an efficient stack-based solution that processes the string in a single pass. Explain the algorithm, analyze its time and space complexity, and discuss edge cases and potential optimizations.
Pro tip: Mention that this is essentially the same as matching parentheses or evaluating expressions, and that a stack naturally handles the 'repeatedly remove' requirement in O(n) time. Also, note that the final string is unique regardless of removal order, which shows deeper insight.
Restate the problem in your own words and walk through a simple example like 'abba' to confirm that removals can cascade. Ask if the input is ASCII or Unicode, and if the string can be empty.
Describe a brute-force method that repeatedly scans the string and removes adjacent pairs until no more exist. Mention its O(n^2) time complexity due to repeated passes.
Explain that a stack can process the string in one pass: for each character, if it matches the top of the stack, pop; otherwise, push. This yields O(n) time and O(n) space.
State the time and space complexity of the stack approach. Discuss edge cases: empty string, no pairs, all pairs, and strings with odd length. Mention that the result is independent of removal order.
If asked, mention that the stack can be simulated with a string builder for O(1) space in some languages, or that the problem can be solved in-place with two pointers. Also, note that the final string is unique.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This is the harder version of the previous question and I underestimated it at first.
Use a stack to process characters one by one, tracking consecutive counts. When the count reaches k, pop the group and merge with the previous stack top if it has the same character, repeating until stable. This yields O(n) time and O(n) space.
Pro tip: Mention that a naive recursive string replacement is O(n^2) or worse, and that the stack approach handles cascading removals in a single pass. Also note edge cases like k=1 (empty string) and k > string length.
Confirm understanding: remove exactly k consecutive identical characters repeatedly until no such group exists. Ask about constraints (string length, character set) and edge cases (k=1, k > length).
Describe a straightforward method: scan for k identical characters, remove them, and repeat until no changes. Note its inefficiency due to repeated scans and string rebuilding.
Explain using a stack of (character, count) pairs. For each character, increment count if it matches the top; otherwise push new. When count reaches k, pop the group and merge with the new top if same character.
Trace the algorithm on a sample like 'deeedbbcccbdaa', k=3 to demonstrate cascading removals and correctness.
State O(n) time and O(n) space. Discuss edge cases: k=1 returns empty string, k > n returns original string, and all characters removed.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.