Flatten a Multilevel Doubly Linked List (Stack)
Flatten a doubly linked list that may contain child pointers. Use a Stack to manage the next level of lists to be processed.
Why Interviewers Ask This
IBM evaluates this question to assess a candidate's mastery of pointer manipulation and recursive thinking in complex data structures. It specifically tests the ability to manage state transitions using an explicit stack, ensuring candidates can handle non-linear linked list topologies without causing memory leaks or infinite loops.
How to Answer This Question
1. Clarify the input: Confirm if the head node is null and define the exact structure of the child pointer. 2. Propose the iterative stack strategy: Explain that you will traverse the main list, pushing 'next' nodes onto a stack whenever a 'child' is encountered, then connecting the current node to its child. 3. Detail the re-linking logic: Describe how to pop from the stack to resume traversal after the child subtree is flattened, ensuring all pointers (prev, next, child) are correctly updated. 4. Discuss edge cases: Mention handling empty lists, nodes with only children, or nested levels deeper than two. 5. Analyze complexity: Explicitly state O(N) time complexity since every node is visited once, and O(N) space for the worst-case stack depth.
Key Points to Cover
- Explicitly mention using a stack to defer processing of 'next' nodes
- Correctly update both 'next' and 'prev' pointers during re-linking
- Ensure the 'child' pointer is set to null after processing
- Analyze time complexity as O(N) and space as O(N)
- Handle the edge case where the input list is empty
Sample Answer
I would approach flattening this multilevel doubly linked list using an iterative method with an explicit stack to ensure clarity and avoid recursion depth issues. First, I initialize a stack and start traversing from th…
Common Mistakes to Avoid
- Forgetting to update the 'prev' pointer of the newly connected child node
- Leaving the 'child' pointer active, which violates the problem constraints
- Using recursion without considering stack overflow risks on deep trees
- Failing to check if the stack is empty before popping, causing runtime errors
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