flattening on Binary Tree
Last updated: February 13, 2026
Quick Overview
Given a binary tree, flatten it into a linked list in-place following the order of pre-order traversal. The output should be the head of the linked list, where each node's right pointer points to the next node in the list and the left pointer is null.
Visa
February 13, 202688
14
2,089 solved
Given a binary tree, flatten it into a linked list in-place following the order of pre-order traversal. The output should be the head of the linked list, where each node's right pointer points to the next node in the list and the left pointer is null.
Visa uses this problem in the Technical Screen 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
- 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 test this solution thoroughly?
- What if the input doesn't fit in memory?
- How would you parallelize this solution?
- What happens if the input contains duplicates?
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Practice DSA ProblemsSample Answer
Problem Analysis
The given problem requires us to flatten a binary tree into a linked list following a pre-order traversal. The key pattern that applies here is the Depth First Search (DFS) approach. This is becau...
Approach
- Start with the root node of the binary tree.
- Create a helper function that takes a node as an argument.
- For each node, save its left and right children.
- Set the current node's left child t...
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