Meta·Software Engineer·Onsite - Coding / Algorithms·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Meta SWE coding round, one problem the whole session. The question looks deceptively simple until you read the constraint about not building the full strings first.

Questions Asked (1)

Given two singly linked lists where each node holds a string, determine whether the full concatenation of the first list equals the full concatenation of the second list. You cannot build the complete strings before comparing; you have to do it incrementally. Target O(total characters) time.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just join everything and compare, which is exactly what the constraint rules out.

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AI HintsAI Generated

Suggested Approach

Use two pointers to traverse both lists simultaneously, comparing characters within nodes and advancing to the next node when a node is exhausted. This simulates concatenation without building strings, achieving O(total characters) time and O(1) extra space.

Pro tip: Clarify edge cases upfront (e.g., empty lists, null nodes, Unicode) and mention that the solution naturally handles them; this shows thoroughness and prevents follow-up traps.

1. Clarify assumptions and edge cases

Ask about empty lists, null nodes, string encodings, and whether lists can be modified. Confirm that the total length is the sum of string lengths.

2. Design two-pointer traversal

Maintain pointers to current nodes and indices within their strings. Compare characters one by one, advancing indices and moving to next nodes when a string is exhausted.

3. Handle termination and mismatches

If characters differ, return false. If both pointers reach the end simultaneously, return true; if one ends before the other, return false.

4. Analyze complexity

Time is O(total characters) since each character is visited once. Space is O(1) beyond input, as only pointers and indices are used.

5. Discuss trade-offs and alternatives

Mention that building strings would be O(total characters) space, which is avoided. Also note that if lists are very long, the incremental approach is memory-efficient.

Key Points to Mention

Two-pointer technique with per-node indices to simulate concatenation.
O(total characters) time and O(1) extra space complexity.
Handling of edge cases: empty lists, null nodes, different lengths.
Avoiding string concatenation to prevent O(n) space usage.
Character-by-character comparison ensures early exit on mismatch.
Potential follow-up: if strings are immutable, accessing characters is O(1).

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