Binary Search on matrix
Last updated: July 15, 2025
Quick Overview
Given a 2D matrix of integers where each row is sorted in ascending order and the first integer of each row is greater than the last integer of the previous row, implement a function to perform a binary search on the matrix to find a target value. The function should return the coordinates of the target as a tuple (row, column) if found, or (-1, -1) if the target is not present in the matrix.
DoorDash
July 15, 20251
5
3,245 solved
Given a 2D matrix of integers where each row is sorted in ascending order and the first integer of each row is greater than the last integer of the previous row, implement a function to perform a binary search on the matrix to find a target value. The function should return the coordinates of the target as a tuple (row, column) if found, or (-1, -1) if the target is not present in the matrix.
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
- 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 parallelize this solution?
- How would you modify your solution to handle streaming input?
- What if the input doesn't fit in memory?
- What is the worst-case input for your solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
The given problem requires us to search for a target value within a 2D matrix that has specific properties: each row is sorted in ascending order, and the first integer of each row is greater than the...
Approach
- Determine the dimensions: First, we need to get the number of rows and columns from the matrix.
- Binary Search Logic: We will use two pointers,
leftandright, initialized to 0 and `...
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