This problem requires flattening a multilevel doubly linked list into a single-level list while preserving order. You must carefully adjust next, prev, and child pointers, inserting any child nodes immediately after their parent. A depth-first traversal is ideal, ensuring all nested children are flattened before moving forward, which avoids common pointer mismanagement errors and produces a correct linear sequence.
Problem Statement
You are given a doubly linked list where each node has next, prev, and an optional child pointer. Some nodes may point to a separate doubly linked list through the child pointer, and these child lists may themselves contain additional children, forming a multilevel structure.
Write a function to flatten the list so that all nodes appear in a single-level doubly linked list. For each node with a child, the nodes in the child list should appear immediately after that node and before its original next node. Ensure all child pointers are set to null in the final flattened list.
Examples
Example 1
Input: head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
Output: [1,2,3,7,8,11,12,9,10,4,5,6]
The multilevel linked list in the input is shown. After flattening the multilevel linked list it becomes:
Example 2
Input: head = [1,2,null,3]
Output: [1,3,2]
The multilevel linked list in the input is shown. After flattening the multilevel linked list it becomes:
Example 3
Input: head = []
Output: []
There could be empty list in the input.
Constraints
- The number of Nodes will not exceed 1000.
- 1 <= Node.val <= 105
Solution Approach
Iterative Depth-First Traversal
Use a stack to traverse the list iteratively. Push next nodes onto the stack before diving into any child nodes, then reconnect pointers as you pop nodes from the stack. This ensures children are flattened before continuing along the original next pointers.
Recursive DFS Flattening
Implement a recursive function that flattens child lists first, then reconnects the current node with the flattened child and the original next node. Return the tail of the flattened list for proper pointer linkage. Recursive DFS simplifies pointer management but uses call stack space proportional to the depth.
Pointer Rewiring Strategy
At each node with a child, identify the child list's head and tail. Connect the current node's next to the child head and the child tail's next to the original next node. Set the child pointer to null after rewiring. This direct pointer manipulation avoids unnecessary stack usage and is optimal for interview clarity.
Complexity Analysis
| Metric | Value |
|---|---|
| Time | Depends on the final approach |
| Space | Depends on the final approach |
Time complexity is O(N), where N is the total number of nodes across all levels, because each node is visited once. Space complexity is O(D) for recursive DFS due to the call stack, where D is the maximum depth of nesting, or O(N) for iterative stack-based approaches in the worst case.
What Interviewers Usually Probe
- Clarify how to handle null child pointers and empty lists.
- Expect you to traverse nodes in the correct flattened order using pointer manipulation.
- Check that all child pointers are properly nullified and prev/next pointers remain consistent.
Common Pitfalls or Variants
Common pitfalls
- Forgetting to set child pointers to null after flattening, leading to dangling references.
- Misconnecting next and prev pointers when inserting child lists, which breaks the doubly linked structure.
- Using inefficient repeated scans instead of a DFS approach, causing higher time complexity.
Follow-up variants
- Flatten a multilevel singly linked list where each node has only next and child pointers.
- Flatten a tree-like linked list with arbitrary branching instead of strict next/child structure.
- Flatten and reverse a multilevel doubly linked list in one pass, maintaining depth-first order.
How GhostInterview Helps
- Provides step-by-step flattening strategies showing how to manage next, prev, and child pointers correctly.
- Highlights DFS and iterative stack methods to avoid pointer mismanagement mistakes during interviews.
- Offers practice on tracing multilevel pointer sequences to ensure the final linearized list is accurate.
Topic Pages
Related GhostInterview Pages
- LeetCode Interview Copilot - Use GhostInterview as a live solver when you want direct help with LeetCode-style coding questions.
- Coding Interview Assistant - See how GhostInterview supports array, string, linked list, graph, and tree interview workflows.
- How GhostInterview Works - Review the screenshot, reasoning, and answer flow before using the solver in a live interview.
FAQ
What is the main pattern used in Flatten a Multilevel Doubly Linked List?
The primary pattern is linked-list pointer manipulation, often implemented with depth-first traversal to flatten child nodes correctly.
Can I flatten the list iteratively instead of recursively?
Yes, using a stack to store next nodes while processing child lists allows an iterative depth-first flattening.
Do I need to handle empty input lists?
Yes, if the head is null, the flattened result should also be null, and no pointer operations are needed.
What happens if a node has multiple nested children?
Each child is recursively flattened and inserted immediately after its parent node, maintaining depth-first order.
How should prev pointers be updated during flattening?
Prev pointers should always point to the previous node in the flattened sequence, including when connecting child list tails to original next nodes.
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