BFS on binary tree
Last updated: March 4, 2026
Quick Overview
Implement a function that performs a Breadth-First Search (BFS) on a binary tree. The function should take the root node of the binary tree as input and return a list of values representing the nodes in the order they are visited during the BFS traversal. Ensure that the solution handles edge cases, such as an empty tree.
Shopify
March 4, 202674
7
4,339 solved
Implement a function that performs a Breadth-First Search (BFS) on a binary tree. The function should take the root node of the binary tree as input and return a list of values representing the nodes in the order they are visited during the BFS traversal. Ensure that the solution handles edge cases, such as an empty tree.
This coding problem is frequently asked during Phone Screen at Shopify. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Shopify expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- What is the worst-case input for your solution?
- How would you parallelize this solution?
- How would you test this solution thoroughly?
- How would your solution change if the input was sorted?
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Practice DSA ProblemsSample Answer
Problem Analysis
This problem requires us to implement a Breadth-First Search (BFS) on a binary tree. The BFS algorithm is appropriate here because it explores the neighbor nodes at the present depth prior to moving o...
Approach
- Initialization: Start with an empty list
resultto store the values of the nodes in BFS order. If the input root isNone, immediately return an empty list, as there are no nodes to traverse...
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