Count longest subsequence in matrix
Last updated: March 8, 2026
Quick Overview
Given a matrix of integers, your task is to find the length of the longest increasing subsequence that can be formed by selecting elements from the matrix in a way that respects their row and column positions. The input will be a 2D array of integers, and the output should be a single integer representing the length of the longest increasing subsequence.
Plaid
March 8, 20264
8
3,568 solved
Given a matrix of integers, your task is to find the length of the longest increasing subsequence that can be formed by selecting elements from the matrix in a way that respects their row and column positions. The input will be a 2D array of integers, and the output should be a single integer representing the length of the longest increasing subsequence.
This coding problem is frequently asked during Technical Screen at Plaid. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Plaid 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?
- How would you test this solution thoroughly?
- How would you parallelize this solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
To solve the problem of finding the longest increasing subsequence in a matrix while respecting row and column positions, we can identify that this is a variation of the Longest Increasing Subsequence...
Approach
- Initialization: Create a 2D array
dpof the same dimensions as the input matrix, initialized to 1. Each element indp[i][j]represents the length of the longest increasing subsequence that ...
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