flattening on Binary Tree
Last updated: May 13, 2026
Quick Overview
Given a binary tree, flatten it into a linked list in-place such that the linked list follows the same order as a pre-order traversal of the tree. The output should be the modified tree where each node's left child is null and the right child points to the next node in the flattened list.
Capital One
May 13, 202647
4
789 solved
Given a binary tree, flatten it into a linked list in-place such that the linked list follows the same order as a pre-order traversal of the tree. The output should be the modified tree where each node's left child is null and the right child points to the next node in the flattened list.
Capital One uses this problem in the Phone 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
- 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
- Can you optimize the space complexity of your solution?
- What is the worst-case input for your solution?
- How would you modify your solution to handle streaming input?
- What happens if the input contains duplicates?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires us to flatten a binary tree into a linked list following the pre-order traversal order. This means we need to visit the root first, then the left subtree, and finally the right su...
Approach
The algorithm can be broken down into the following steps:
- Start with the root of the binary tree.
- Use a recursive DFS function that processes each node:
- Save the left and right children of...
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