Find longest subsequence in grid
Last updated: September 14, 2025
Quick Overview
Given a grid of characters, write a function to find the longest subsequence that can be formed by traversing the grid in any direction (up, down, left, right) without revisiting any cell. The function should return the length of this longest subsequence. The input will be a 2D array of characters, and the output will be an integer representing the length of the longest subsequence found.
Apple
September 14, 202541
3
170 solved
Given a grid of characters, write a function to find the longest subsequence that can be formed by traversing the grid in any direction (up, down, left, right) without revisiting any cell. The function should return the length of this longest subsequence. The input will be a 2D array of characters, and the output will be an integer representing the length of the longest subsequence found.
This coding problem is frequently asked during Phone Screen at Apple. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Apple expects candidates to write production-quality code, not just solve the puzzle.
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?
- How would your solution change if the input was sorted?
- How would you modify your solution to handle streaming input?
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Problem Analysis
To solve the problem of finding the longest subsequence in a grid of characters, we can recognize that this is a Depth First Search (DFS) problem. The key points are:
- Traversal Directions: We ...
Approach
Here's a step-by-step algorithm to find the longest subsequence:
- Initialize Variables: Declare a variable to keep track of the maximum length of the subsequence found. Create a visited set to t...
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