flattening on Binary Tree
Last updated: July 31, 2025
Quick Overview
Perform a partitioning on a binary tree and return the result.
ServiceNow
Coding & Algorithms
Software Engineer
ServiceNow
July 31, 2025Software Engineer
Onsite
Coding & Algorithms
Hard
20
13
2,245 solved
Perform a partitioning on a binary tree and return the result.
ServiceNow uses this problem in the Onsite 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
- 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
Edge cases and input validation
Tree structures and recursion
Time and space complexity analysis
Hash maps and frequency counting
Sorting and searching
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 optimize the space complexity of your solution?
- Can you solve this in a single pass?
- What is the worst-case input for your solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
To flatten a binary tree, we need to convert it into a linked list in-place. The specific pattern here is a preorder traversal of the tree, as we need to visit the root before its children. This a...
Approach
- Start with a recursive function that takes a node as input. If the node is null, return immediately.
- Store the left and right children of the current node.
- Recursively call the function for...
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