Transform matrix to hash map
Last updated: March 1, 2026
Quick Overview
Given a 2D matrix, transform it into a hash map where the keys are the unique elements from the matrix and the values are the counts of those elements. The output should be a hash map that accurately reflects the frequency of each element in the matrix. Ensure that the solution efficiently handles large matrices.
DoorDash
March 1, 20267
15
1,817 solved
Given a 2D matrix, transform it into a hash map where the keys are the unique elements from the matrix and the values are the counts of those elements. The output should be a hash map that accurately reflects the frequency of each element in the matrix. Ensure that the solution efficiently handles large matrices.
This coding problem is frequently asked during Phone Screen at DoorDash. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. DoorDash 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
- Can you solve this in a single pass?
- What is the worst-case input for your solution?
- How would you modify your solution to handle streaming input?
- Can you solve this iteratively instead of recursively (or vice versa)?
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Problem Analysis
To solve the problem of transforming a 2D matrix into a hash map where keys are unique elements and values are their counts, we can employ a dictionary (hash map) in Python. This approach is effic...
Approach
- Initialize an empty dictionary to hold our counts.
- Iterate through each row of the matrix:
- For each row, iterate through each element:
- If the element is already in the dictionary, in...
- For each row, iterate through each element:
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