flattening on Binary Tree
Last updated: May 17, 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 linked list formed by the flattened tree, where each node's right pointer points to the next node in the list and the left pointer is set to null.
Vercel
May 17, 2026254
13
2,572 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 linked list formed by the flattened tree, where each node's right pointer points to the next node in the list and the left pointer is set to null.
This coding problem is frequently asked during Take-home Project at Vercel. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Vercel expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Identify the correct data structure and algorithm for the problem
- Write clean, bug-free code with proper variable naming
- Analyze time and space complexity correctly
- Handle basic edge cases (empty input, single element)
- Communicate your thought process while coding
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
- What happens if the input contains duplicates?
- How would you test this solution thoroughly?
- How would you modify your solution to handle streaming input?
- Can you solve this in a single pass?
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Problem Analysis
To flatten a binary tree into a linked list following a pre-order traversal, we should recognize that we can use a Depth-First Search (DFS) approach. The pre-order traversal visits nodes in the order:...
Approach
- Start from the root of the binary tree.
- For each node, first visit the node and link it to the last visited node.
- Recursively traverse the left subtree, then the right subtree.
- While...
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