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Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026Remote

Summary

Meta SWE coding round with a tree traversal problem. Pretty standard algorithmic stuff but the column ordering logic tripped me up more than I expected.

Questions Asked (1)

Q1

Given a binary tree, implement a function that returns node values grouped by vertical column, ordered left to right and top to bottom within each column.

Algorithms & Data Structures
Author's notes

The example looks simple enough until you actually sit down to implement it.

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AI HintsAI Generated

Suggested Approach

Use a BFS traversal while tracking each node's horizontal distance (column index) from the root. Store nodes in a hash map keyed by column, then sort the columns and output the values in order.

Pro tip: Mention that BFS naturally preserves top-to-bottom order within each column, and for columns with the same horizontal distance, nodes are visited left-to-right due to the queue order. Also, note that using a TreeMap can avoid explicit sorting.

1. Define the column index

Assign the root column index 0. For each left child, decrement the parent's column index by 1; for each right child, increment by 1.

2. Traverse the tree

Perform a BFS (level-order) traversal using a queue. Store pairs of (node, column index) in the queue.

3. Group nodes by column

Use a hash map (or TreeMap) to map each column index to a list of node values. Append values as nodes are dequeued.

4. Order and output

If using a hash map, sort the keys (column indices) in ascending order. Then output the lists of values for each column in that order.

Key Points to Mention

  • BFS ensures top-to-bottom order within each column.
  • Horizontal distance concept: left child = parent column - 1, right child = parent column + 1.
  • Using a TreeMap automatically keeps columns sorted, avoiding a separate sorting step.
  • Time complexity: O(N log N) if sorting columns, O(N) with TreeMap; space complexity: O(N).
  • Edge cases: empty tree, skewed tree, nodes with same column but different levels.
  • Alternative: DFS with column tracking also works, but BFS is more intuitive for level order.

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