Count longest subsequence in binary tree
Last updated: February 21, 2026
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's value. The function should take the root of the binary tree as input and return an integer representing the length of the longest increasing subsequence found in the tree.
Capital One
February 21, 202611
13
636 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's value. The function should take the root of the binary tree as input and return an integer representing the length of the longest increasing subsequence found in the tree.
Capital One uses this problem in the Onsite 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 your solution change if the input was sorted?
- What happens if the input contains duplicates?
- What is the worst-case input for your solution?
- 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 counting the longest subsequence in a binary tree where each node's value is greater than its parent's value, we can utilize a Depth-First Search (DFS) traversal pattern. The r...
Approach
- We will define a recursive function that takes a node and the current length of the increasing subsequence as parameters.
- If the node is null, we will return the length of the current subsequen...
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