Write L Matrix
Reported by candidates from DataBricks's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Databricks asked candidates in September 2024 to write an L matrix, a problem that sounds simple until you realize it's testing your ability to traverse and construct data structures in a specific order. You're likely seeing this because you have the OA in the next day or two, and you need to know if this is a BFS trap or something else. The trick is understanding what "L" means in their context and how to build it systematically without losing your way on the actual platform.
Pattern and pitfall
The L matrix problem almost certainly involves constructing a matrix by reading input in a breadth-first order, or outputting an existing matrix in BFS traversal order. The common mistake is confusing row-major/column-major iteration with actual BFS, or building the matrix row by row when you should be thinking in layers or levels. If you're building from BFS input, you need to track indices carefully and avoid off-by-one errors. StealthCoder becomes your safety net here because the indexing logic can feel abstract in the moment, and a real-time hint on the exact BFS traversal order or matrix construction logic can stop you from rewriting code three times under time pressure.
If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.
You can drill Write L Matrix cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as binary tree level order traversal. If you have time before the OA, drill that.
You've seen the question.
Make sure you actually pass DataBricks's OA.
DataBricks reuses patterns across OAs. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Write L Matrix FAQ
Is this actually a BFS problem or just confusingly named?+
It's likely BFS-adjacent. The 'L' probably refers to level-order, layer-by-layer, or a specific traversal. If you're constructing or reading a matrix in level order, that's BFS. If it's just printing a matrix in a weird order, it's still BFS. Don't overthink the name.
What's the most common mistake on this one?+
Mixing up iteration order. Candidates often iterate row-by-row when the problem wants level-order, or they don't track the queue indices correctly. Read the exact expected output format first. That tells you the traversal order.
How do I prepare for this in 48 hours if I've never seen it?+
Practice LeetCode 102 (binary tree level-order traversal) and understand how a queue-based BFS works. Then think: if this were a matrix instead of a tree, how would I apply the same logic. Write pseudo-code first.
Do I need to handle matrix dimensions dynamically?+
Very likely. Databricks problems often give you variable-sized inputs. Assume the matrix size is not hard-coded. Read dimensions from input and allocate accordingly. Don't assume square matrices unless stated.
Is this problem still asked at Databricks in late 2024?+
Yes, it was reported in September 2024. It's stable in their rotation. BFS and matrix problems are core patterns for them. If you see 'matrix' and 'traverse' in the problem statement, assume BFS or DFS is the answer.