Validate binary tree complete
Last updated: July 12, 2025
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.
Booking.com
July 12, 2025122
4
570 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 Phone Screen at Booking.com. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Booking.com expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
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 you modify your solution to handle streaming input?
- Can you solve this iteratively instead of recursively (or vice versa)?
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Problem Analysis
To determine if a binary tree is complete, we can leverage a level-order traversal (BFS) approach. A complete binary tree requires that all levels are fully filled except possibly the last, and if the...
Approach
- Initialize a queue: Start with the root node and enqueue it.
- Process nodes in the queue: While the queue is not empty, dequeue a node and check its children:
- If a node has a right c...
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