Optimize flattening for in-place
Last updated: November 17, 2025
Quick Overview
Given a binary tree, implement an in-place algorithm to flatten it into a linked list following the same order as a pre-order traversal. The output should modify the tree structure directly without using any additional data structures, and the linked list should be formed by rearranging the tree nodes.
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Given a binary tree, implement an in-place algorithm to flatten it into a linked list following the same order as a pre-order traversal. The output should modify the tree structure directly without using any additional data structures, and the linked list should be formed by rearranging the tree nodes.
This coding problem is frequently asked during Onsite at Google. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Google 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 optimize the space complexity of your solution?
- How would you test this solution thoroughly?
- How would you modify your solution to handle streaming input?
- What is the worst-case input for your solution?
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Problem Analysis
To flatten a binary tree into a linked list in-place following pre-order traversal, we can utilize a depth-first search (DFS) approach. The key observation is that during pre-order traversal, we visit...
Approach
- Initialize: Start with the root node of the binary tree.
- Traverse: Use a recursive DFS function that takes a node as input.
- If the current node is null, return.
- Store the right...
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