I knew merge sort conceptually but applying it to a doubly linked list tripped me up more than I expected.
Start by clarifying the problem and constraints, then explain the merge sort algorithm adapted for a doubly linked list. Emphasize the key steps: finding the middle using fast/slow pointers, recursively sorting each half, and merging two sorted doubly linked lists while maintaining prev/next pointers. Walk through the code and analyze time and space complexity.
Pro tip: Mention that merge sort is preferred for linked lists because it doesn't require random access and can be done with O(1) extra space if implemented iteratively, unlike arrays. Also, highlight the importance of correctly updating both next and prev pointers during merge to avoid breaking the list.
Ask clarifying questions (e.g., in-place, stability, memory constraints) and outline the approach: divide the list into halves, recursively sort, and merge.
Use the fast and slow pointer technique to find the middle node. Ensure the list is split into two halves by disconnecting the prev pointer of the second half's head.
Recursively apply merge sort to the left and right halves until each sublist has one node (base case).
Merge two sorted doubly linked lists by comparing nodes and adjusting next and prev pointers. Use a dummy node to simplify the merge process.
State that time complexity is O(n log n) and space complexity is O(log n) due to recursion stack (or O(1) if iterative merge is used).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.