How do you flatten a linked list with random pointers?
This advanced problem tests your ability to handle complex linked list structures with additional pointers beyond the standard next pointer.
Why Interviewers Ask This
It challenges candidates to think about modifying structures in-place without losing references. Interviewers evaluate recursive vs iterative approaches and memory management skills. It also tests robustness against circular references or complex branching.
How to Answer This Question
Clarify the structure of the linked list (e.g., multilevel). Propose a depth-first search approach. Explain how to link child nodes to the main chain. Discuss preserving the original order and handling null pointers carefully. Compare space complexities of recursive and iterative solutions.
Key Points to Cover
- Recursive traversal strategy
- Preserving next pointers
- In-place modification
- Handling null children
Sample Answer
I will traverse the list recursively. When encountering a child node, I save the next pointer, connect the current node to the child, and recursively flatten the child. After flattening, I attach the saved next pointer to the end of the flattened child list. This maintains the linear structure.
Common Mistakes to Avoid
- Losing track of the next node
- Creating cycles accidentally
- Not flattening nested levels fully
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