Merge K Sorted binary trees
Last updated: October 24, 2025
Quick Overview
Given k sorted strings, merge them into a single sorted result.
Elastic
October 24, 202519
4
3,776 solved
Given k sorted strings, merge them into a single sorted result.
This coding problem is frequently asked during Onsite at Elastic. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Elastic expects candidates to write production-quality code, not just solve the puzzle.
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 iteratively instead of recursively (or vice versa)?
- How would your solution change if the input was sorted?
- Can you solve this in a single pass?
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Practice DSA ProblemsSample Answer
Problem Analysis
To merge k sorted binary trees, we can leverage a priority queue (min-heap) to efficiently combine the trees. The reason a priority queue is ideal here is that it allows us to always access the smalle...
Approach
- Initialize a Min-Heap: Start by creating a min-heap (priority queue) to hold the roots of the k trees.
- Insert All Roots: Insert the root nodes of all k trees into the min-heap.
- **Merg...
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