← Jane Street Interview Insights
This one took me a while to even understand what they were asking.
Model each diff as a sequence of insertions at original document positions, then merge them by simulating the combined effect on positions without materializing the document. Use a two-pointer technique to process both diffs in order, adjusting positions in the second diff by the cumulative offset introduced by the first diff. Output the merged diff as a sorted list of position-value pairs.
Pro tip: Clarify whether positions are 0-indexed or 1-indexed and whether insertions at the same position preserve order; these details affect the merge logic and demonstrate attention to edge cases.
Confirm the meaning of a diff (sorted list of (position, value) insertions), indexing base, and how ties at the same position are ordered. Establish that the merged diff must be equivalent to applying the first then the second diff.
Use two pointers to traverse both diffs. For each insertion in the second diff, compute its effective position in the original document by subtracting the number of insertions from the first diff that occur at or before that position. Merge the two sorted lists of adjusted insertions.
When merging, ensure that insertions from the first diff and adjusted insertions from the second diff are interleaved correctly. If positions tie, decide the order based on the original application order (first diff before second diff).
Explain that the algorithm runs in O(n+m) time because each diff is traversed once, and uses O(n+m) space for the output. Emphasize that no document is constructed or iterated.
Walk through examples: empty diffs, insertions at the same position, insertions at the beginning/end, and interleaved insertions. Verify that the merged diff produces the same final document as applying the diffs sequentially.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.