flattening on Binary Tree
Last updated: March 28, 2026
Quick Overview
Given a binary tree, flatten it into a linked list in-place following the order of the tree's nodes. The left child of each node should be null, and the right child should point to the next node in the flattened list. Your function should return the root of the modified tree.
Square/Block
March 28, 20262
14
2,075 solved
Given a binary tree, flatten it into a linked list in-place following the order of the tree's nodes. The left child of each node should be null, and the right child should point to the next node in the flattened list. Your function should return the root of the modified tree.
Coding interviews at Square/Block 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 parallelize this solution?
- Can you solve this in a single pass?
- What is the worst-case input for your solution?
- What if the input doesn't fit in memory?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires us to flatten a binary tree into a linked list in-place. The specific pattern that applies here is Depth-First Search (DFS) because we need to explore each node and its children r...
Approach
- Start with a helper function that takes the current node as input.
- Recursively call this helper function on the left and right children of the current node.
- After the recursive calls, connect...
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