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Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Meta SWE coding round, got a binary tree problem that looks straightforward until you actually sit down to implement it. BFS with column tracking is the move but I second-guessed myself halfway through.

Questions Asked (1)

Q1

Given a binary tree, return its vertical order traversal where nodes are grouped by column index (root at 0, left child at -1, right child at +1), ordered left to right by column and top to bottom within each column.

Algorithms & Data Structures
Author's notes

I knew BFS was the right approach but spent too long debating whether to use DFS first.

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AI HintsAI Generated

Suggested Approach

Use a BFS traversal while tracking each node's column index, storing nodes in a hash map keyed by column. After traversal, sort the columns and output nodes in the order they were visited within each column to ensure top-to-bottom ordering.

Pro tip: Clarify whether nodes in the same column and row should be ordered left-to-right by their parent's column; if so, use a min-heap or sort by row and value. Also, mention that BFS naturally preserves top-to-bottom order, but if using DFS, you must track depth to sort correctly.

1. Clarify requirements and edge cases

Ask about ordering within the same column and row, and confirm handling of empty tree or single node. Discuss whether to return a list of lists or another format.

2. Choose traversal and data structures

Select BFS (level-order) with a queue storing (node, column) pairs, and a hash map (dictionary) mapping column index to a list of node values. Alternatively, use DFS with depth tracking if preferred.

3. Traverse and populate the map

Process nodes level by level, updating the column index for children (-1 for left, +1 for right). Append node values to the corresponding column list in the map.

4. Sort and format output

Extract keys from the map, sort them in ascending order, and build the result list by iterating through sorted columns. Ensure nodes within each column are in top-to-bottom order (BFS naturally provides this).

5. Analyze complexity and test

State time complexity O(N log N) due to sorting columns (or O(N) if using ordered map), and space O(N). Walk through a small example to verify correctness.

Key Points to Mention

  • Use BFS to guarantee top-to-bottom ordering within each column.
  • Track column indices relative to the root (root at 0).
  • Store nodes in a hash map with column index as key.
  • Sort column indices before outputting to ensure left-to-right order.
  • Handle edge cases: empty tree, skewed tree, nodes with same column and row.
  • Time complexity: O(N log N) due to sorting columns; space O(N).

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