flattening on Binary Tree
Last updated: May 3, 2026
Quick Overview
Given a binary tree, flatten it into a linked list in-place following the same order as a pre-order traversal. The output should be the head of the flattened linked list, where each node's left child is null and the right child points to the next node in the list.
Apple
May 3, 20264
5
3,849 solved
Given a binary tree, flatten it into a linked list in-place following the same order as a pre-order traversal. The output should be the head of the flattened linked list, where each node's left child is null and the right child points to the next node in the list.
Coding interviews at Apple focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
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 you modify your solution to handle streaming input?
- How would your solution change if the input was sorted?
- Can you optimize the space complexity of your solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
We need to flatten a binary tree into a linked list following a pre-order traversal. The key pattern here is utilizing a depth-first search (DFS) approach. This is because pre-order traversal visits n...
Approach
- Start from the root of the binary tree.
- Use a helper function that performs a DFS traversal. For each node:
- Save the left and right children.
- Set the left child of the current node to ...
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