Validate binary tree complete
Last updated: June 5, 2026
Quick Overview
Given a binary tree, write a function to determine if it is a complete binary tree. A complete binary tree is defined as every level, except possibly the last, being completely filled, and all nodes are as far left as possible. The function should return a boolean value: true if the tree is complete, and false otherwise.
Airbnb
June 5, 2026136
13
2,735 solved
Given a binary tree, write a function to determine if it is a complete binary tree. A complete binary tree is defined as every level, except possibly the last, being completely filled, and all nodes are as far left as possible. The function should return a boolean value: true if the tree is complete, and false otherwise.
This coding problem is frequently asked during Onsite at Airbnb. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Airbnb 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
- Can you solve this in a single pass?
- Can you optimize the space complexity of your solution?
- What is the worst-case input for your solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
To determine if a binary tree is complete, we can use a level-order traversal approach, which is best suited for this problem. The reason is that a complete binary tree requires all levels to be fully...
Approach
- Use a queue to perform a level-order traversal of the binary tree.
- Initialize a variable
endto track whether we have encountered a null node. This helps in identifying if subsequent nodes are...
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