Bloomberg·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Bloomberg SWE interview with a linked list problem that sounds manageable until you actually try to implement it cleanly under pressure.

Questions Asked (1)

You're given a doubly linked list where each node can have a child pointer leading to another doubly linked list, possibly nested multiple levels deep. Flatten the entire structure into a single doubly linked list where each child list is inserted immediately after its parent node and before the parent's original next node. All child pointers should be null in the result.

Algorithms & Data Structures

Author's notes

I got the high-level idea pretty fast, like yeah just splice the child list in after the current node and keep going.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Traverse the list iteratively, and whenever a node has a child, flatten the child list recursively and splice it between the current node and its next node. Update all prev and next pointers accordingly and set child to null.

Pro tip: Clarify whether the flattening should be done in-place or if a new list can be created; in-place is typically expected and more efficient. Also, mention that recursion depth could be an issue for very deep nesting, so an iterative stack-based approach might be preferable.

1. Understand the structure and requirements

Confirm that each node has next, prev, and child pointers, and that the goal is to produce a single-level doubly linked list with child pointers set to null.

2. Choose traversal strategy

Decide between recursive depth-first traversal or iterative stack-based traversal to handle nested lists without losing track of the parent's next node.

3. Flatten and splice child lists

When encountering a node with a child, flatten the child list, then connect the current node's next to the child's head, and the child's tail to the saved next node, updating prev pointers.

4. Update pointers and clear child

Ensure all prev and next pointers are correctly set, and set the child pointer of the parent node to null after splicing.

5. Handle edge cases and verify

Test with empty list, single node, multiple levels of nesting, and ensure no cycles or broken links; walk through the list to confirm correctness.

Key Points to Mention

Time complexity: O(N) where N is total number of nodes, as each node is visited once.
Space complexity: O(1) if iterative with no extra stack, or O(D) for recursion depth D.
Need to save the next node before overwriting pointers to avoid losing the rest of the list.
Updating prev pointers is crucial for maintaining doubly linked list integrity.
Setting child to null after processing to meet the requirement.
Consider iterative approach with a stack to avoid recursion depth limits for deeply nested lists.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.