flattening on Binary Tree
Last updated: December 13, 2025
Quick Overview
Given a binary tree, flatten the tree into a linked list in-place, where the left child of each node is set to null and the right child points to the next node in the preorder traversal order. The function should modify the tree structure directly and not return any value.
Palantir
December 13, 2025107
10
3,431 solved
Given a binary tree, flatten the tree into a linked list in-place, where the left child of each node is set to null and the right child points to the next node in the preorder traversal order. The function should modify the tree structure directly and not return any value.
This coding problem is frequently asked during Technical Screen at Palantir. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Palantir 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
- How would your solution change if the input was sorted?
- Can you solve this iteratively instead of recursively (or vice versa)?
- Can you solve this in a single pass?
- How would you modify your solution to handle streaming input?
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Problem Analysis
To flatten a binary tree into a linked list in-place, we can recognize that the problem aligns with a depth-first traversal pattern, specifically preorder traversal (root, left, right). The goal is to...
Approach
- Define a helper function that will perform the preorder traversal recursively.
- During the traversal, maintain a variable
prevthat tracks the last node visited. - For each node visited: -...
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