Meta·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026

Summary

Meta ML engineer coding round, got a linked list problem that looked straightforward until the random pointer showed up. They wanted two full solutions with complexity analysis, not just one.

Questions Asked (1)

Given a linked list where each node has both a next pointer and a random pointer (pointing to any node or null), implement a deep copy of the list. They want two approaches: one using a hashmap for O(n) space, and another using O(1) extra space by interleaving copy nodes into the original list before separating them out. Walk through the complexity and trade-offs of both.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The hashmap version I got out pretty cleanly.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then present the hashmap approach with O(n) space, followed by the interleaving approach with O(1) extra space. For each, walk through the algorithm step-by-step, analyze time and space complexity, and discuss trade-offs such as code simplicity, mutation of input, and practical considerations.

Pro tip: Emphasize that the O(1) space approach temporarily modifies the original list, which may be unacceptable in concurrent or immutable settings; always ask if input mutation is allowed. Also, mention that the hashmap approach is often preferred in practice for its simplicity and safety, despite higher space usage.

1. Clarify requirements and edge cases

Ask about input mutation constraints, list size, and whether null pointers are allowed. Discuss edge cases like empty list, single node, and random pointers to null.

2. Present hashmap approach

Explain mapping original nodes to their copies in a hashmap, then setting next and random pointers for copies using the map. Analyze O(n) time and O(n) space.

3. Present interleaving approach

Describe inserting copy nodes after each original node, setting random pointers for copies, then separating the two lists. Analyze O(n) time and O(1) extra space.

4. Compare trade-offs

Discuss space vs. simplicity, input mutation, and suitability for production. Mention that the interleaving approach is more complex and error-prone but meets strict space constraints.

5. Summarize and conclude

Reiterate the two approaches, their complexities, and when to use each. Offer to code either approach if needed.

Key Points to Mention

Time complexity: both approaches are O(n) time.
Space complexity: hashmap uses O(n) extra space; interleaving uses O(1) extra space.
Hashmap approach: simple, does not modify original list, but uses extra memory.
Interleaving approach: modifies original list temporarily, requires careful pointer manipulation, but constant space.
Trade-offs: mutation may be problematic in concurrent or immutable contexts; hashmap is safer and easier to maintain.
Edge cases: empty list, single node, random pointers to null or to any node.

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