Count longest subsequence in binary tree
Last updated: May 3, 2026
Quick Overview
Given a binary tree, write a function to count the length of the longest subsequence of nodes where each node's value is either 0 or 1. The function should return an integer representing the length of this longest subsequence. The input will be the root node of the binary tree, and the output should be the maximum length of the valid subsequence found.
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Given a binary tree, write a function to count the length of the longest subsequence of nodes where each node's value is either 0 or 1. The function should return an integer representing the length of this longest subsequence. The input will be the root node of the binary tree, and the output should be the maximum length of the valid subsequence found.
This coding problem is frequently asked during Technical Screen at Reddit. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Reddit expects candidates to write production-quality code, not just solve the puzzle.
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
- How would you modify your solution to handle streaming input?
- Can you solve this in a single pass?
- What is the worst-case input for your solution?
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Problem Analysis
This problem can be effectively solved using Depth-First Search (DFS) due to its recursive nature in exploring the binary tree. The goal is to count the longest subsequence of nodes where each node's ...
Approach
- Start at the root of the binary tree and initialize a variable to keep track of the maximum length of valid subsequences found.
- Use a recursive DFS function that takes the current node and the c...
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