Binary Tree Level Order Traversal II (Bottom-up)
Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. Use BFS and reverse the final result.
Why Interviewers Ask This
Interviewers at Airbnb ask this to verify your mastery of Breadth-First Search (BFS) and queue manipulation. They specifically test if you can handle level-by-level processing and understand how to reverse the output order efficiently without compromising time complexity. This assesses your ability to solve standard tree problems with a slight variation, ensuring you don't just memorize solutions but adapt algorithms logically.
How to Answer This Question
1. Clarify requirements: Confirm input is a root node and define 'bottom-up' as starting from the deepest leaf level up to the root. 2. Select Data Structure: Explicitly state you will use a Queue for BFS to process nodes level by level, ensuring O(N) time complexity. 3. Implement Standard BFS: Describe iterating through the current queue size to isolate each level, collecting values into a temporary list. 4. Handle Reversal: Explain that instead of complex logic during traversal, you will simply reverse the final list of lists after the loop completes. 5. Analyze Complexity: Conclude by stating the solution runs in O(N) time and O(N) space, referencing Airbnb's focus on scalable performance even for easy questions.
Key Points to Cover
- Explicitly mentioning the use of a Queue for BFS demonstrates fundamental algorithmic knowledge
- Explaining the level-by-level iteration logic proves understanding of tree traversal mechanics
- Choosing to reverse the list post-traversal shows awareness of efficient coding practices over complex insertion logic
- Stating O(N) time and space complexity confirms ability to analyze algorithmic performance
- Clarifying edge cases like empty trees or single-node trees shows attention to detail
Sample Answer
To solve the bottom-up level order traversal, I would first clarify that we need to return levels starting from the leaves up to the root. My approach involves using a standard Breadth-First Search strategy with a queue…
Common Mistakes to Avoid
- Using recursion which leads to stack overflow risks on deep trees instead of iterative BFS
- Attempting to insert each level at the beginning of the result list causing O(N^2) time complexity
- Failing to handle null root inputs or empty trees resulting in runtime errors
- Confusing bottom-up traversal with pre-order or post-order depth-first search approaches
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