Transform matrix to hash map
Last updated: September 24, 2025
Quick Overview
Given a matrix of integers, transform it into a hash map where the keys are the unique integers from the matrix and the values are the counts of their occurrences. The input will be a 2D array of integers, and the output should be a hash map (or dictionary) representing the frequency of each integer. Ensure that the solution efficiently handles large matrices.
Grubhub
September 24, 202519
10
4,520 solved
Given a matrix of integers, transform it into a hash map where the keys are the unique integers from the matrix and the values are the counts of their occurrences. The input will be a 2D array of integers, and the output should be a hash map (or dictionary) representing the frequency of each integer. Ensure that the solution efficiently handles large matrices.
This coding problem is frequently asked during Technical Screen at Grubhub. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Grubhub 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
- How would you parallelize this solution?
- How would you modify your solution to handle streaming input?
- What is the worst-case input for your solution?
Sharpen Your Skills on Codemia
Practice similar problems with our interactive workspace, get AI feedback, and track your progress.
Practice DSA ProblemsSample Answer
Problem Analysis
This problem can be efficiently solved using a hash map (or dictionary in Python) to count the occurrences of each integer in the matrix. The key pattern here is frequency counting, which allows us to...
Approach
- Initialize an empty dictionary to hold the frequency counts.
- Iterate over each row of the matrix.
- For each element within the row, check if it exists in the dictionary:
- If it does, i...
- For each element within the row, check if it exists in the dictionary:
Submit Your Answer
