flattening on Binary Tree
Last updated: July 16, 2025
Quick Overview
Given a binary tree, flatten it into a linked list in-place such that the linked list nodes follow the same order as a pre-order traversal of the tree. The output should be the root of the modified tree, where the left child of each node is null and the right child points to the next node in the list.
Snapchat
July 16, 2025203
14
4,879 solved
Given a binary tree, flatten it into a linked list in-place such that the linked list nodes follow the same order as a pre-order traversal of the tree. The output should be the root of the modified tree, where the left child of each node is null and the right child points to the next node in the list.
Snapchat uses this problem in the Take-home Project 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
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
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?
- Can you solve this in a single pass?
- What happens if the input contains duplicates?
- How would your solution change if the input was sorted?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires us to flatten a binary tree into a linked list following a pre-order traversal. The key observation here is that we can utilize a pre-order traversal, which visits nodes in th...
Approach
Our approach involves using a recursive traversal. The steps are as follows:
- Perform a pre-order traversal of the binary tree.
- For each node visited, we need to set its left child to
null...
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