Count longest subsequence in binary tree
Last updated: December 21, 2025
Quick Overview
Given a binary tree, write a function to count the length of the longest subsequence where each node's value is greater than its parent node's value. The function should take the root node of the binary tree as input and return an integer representing the length of the longest increasing subsequence found.
Zillow
December 21, 20255
11
3,431 solved
Given a binary tree, write a function to count the length of the longest subsequence where each node's value is greater than its parent node's value. The function should take the root node of the binary tree as input and return an integer representing the length of the longest increasing subsequence found.
Coding interviews at Zillow focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
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
- What is the worst-case input for your solution?
- What if the input doesn't fit in memory?
- How would you modify your solution to handle streaming input?
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Problem Analysis
To solve the problem of counting the longest subsequence in a binary tree where each node's value is greater than its parent node's value, we can apply a Depth-First Search (DFS) approach. This is bec...
Approach
- Start at the root node of the binary tree.
- Initialize a recursive function that takes the current node and the length of the current increasing subsequence as arguments.
- For each node:
- I...
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