Bloomberg·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Bloomberg SWE interview with a linked list problem that looks manageable until you actually try to get the pointer reconnection right. The recursive child-flattening part is where things get fiddly.

Questions Asked (1)

Given a doubly linked list where nodes can have a child pointer referencing another doubly linked list (nested arbitrarily deep), flatten it into a single-level doubly linked list. Child lists should be inserted immediately after the node that holds them, and all child pointers must be nulled out with prev/next pointers correctly updated throughout.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I got the high-level idea pretty fast: depth-first, splice the child in, move on.

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AI HintsAI Generated

Suggested Approach

Traverse the list iteratively, and when a node with a child is encountered, splice the entire child list between the current node and its next node. Use a stack to defer processing of the original next node, or recursively flatten the child list before continuing. Ensure all child pointers are set to null and prev/next pointers are correctly updated.

Pro tip: Clarify whether the flattening should be done in-place and discuss the trade-offs between iterative (using a stack) and recursive approaches, especially regarding stack overflow for deep nesting. Mention that you can achieve O(1) space with a clever pointer manipulation if you flatten depth-first without a stack by reusing the child pointers.

1. Understand the structure and requirements

Confirm that each node has prev, next, and child pointers, and that child lists are also doubly linked. The goal is to produce a single-level doubly linked list where child lists are inserted immediately after their parent node, and all child pointers become null.

2. Choose an approach

Decide between iterative (using a stack) and recursive approaches. Iterative with a stack avoids recursion depth limits; recursive is simpler but may overflow for deep nesting. Discuss trade-offs.

3. Traverse and splice

Traverse the list. When a node with a child is found, save its next node, then insert the child list between the current node and the saved next node. Set the child pointer to null. Continue traversal into the child list, and later resume from the saved next node.

4. Maintain pointers correctly

Ensure that prev and next pointers are updated for all affected nodes: the current node's next becomes the child's head, the child's head's prev becomes the current node, the child's tail's next becomes the saved next node, and the saved next node's prev becomes the child's tail.

5. Handle edge cases and verify

Consider empty list, no child pointers, child at the end of the list, and multiple nested levels. Walk through a small example to verify correctness and pointer integrity.

Key Points to Mention

Time complexity: O(N) where N is total number of nodes across all levels, as each node is visited once.
Space complexity: O(1) if using iterative in-place splicing without a stack, or O(D) for recursion/stack where D is maximum depth.
In-place modification: no new nodes are created; only pointers are rearranged.
Nulling child pointers: essential to avoid cycles and meet the requirement.
Pointer updates: careful handling of prev and next for head and tail of child lists.
Edge cases: empty list, single node, child at tail, deeply nested lists.

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