Find longest subsequence in binary tree
Last updated: October 22, 2025
Quick Overview
Given a binary tree, write a function to find the longest subsequence of nodes where each node's value is greater than its parent node's value. The function should return the length of this longest subsequence. The input will be the root node of the binary tree, and the output should be an integer representing the length of the longest increasing subsequence.
Datadog
October 22, 20250
11
50 solved
Given a binary tree, write a function to find the longest subsequence of nodes where each node's value is greater than its parent node's value. The function should return the length of this longest subsequence. The input will be the root node of the binary tree, and the output should be an integer representing the length of the longest increasing subsequence.
Datadog 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
- Can you optimize the space complexity of your solution?
- Can you solve this in a single pass?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
To solve the problem of finding the longest subsequence of nodes in a binary tree where each node's value is greater than its parent node's value, we can identify that this is a tree traversal problem...
Approach
- DFS Traversal: We will use a recursive DFS approach to traverse the binary tree.
- Track Length: For each node, we will compare its value with its parent's value. If the current node's va...
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