Validate binary tree complete
Last updated: November 24, 2025
Quick Overview
Given a binary tree, implement 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.
Datadog
November 24, 20255
6
482 solved
Given a binary tree, implement 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.
Datadog uses this problem in the Phone Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
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?
- How would your solution change if the input was sorted?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
To determine if a binary tree is complete, we can use the Level Order Traversal approach, which utilizes a Breadth-First Search (BFS) pattern. This applies here because a complete binary tree ...
Approach
- Start with a queue initialized with the root of the binary tree.
- Perform a level order traversal using the queue:
- Dequeue a node and check if it's
null. - If we encounter a
nullnode...
- Dequeue a node and check if it's
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