Top 10 Matrix Coding Exercises for Programming interviews and Homework

Hello guys, if you are preparing for Coding interviews and solved 100+ data structure problems then you may know that Matrix is one of the under-rated topic for coding interviews but Matrix coding problems are not that easy to solve and  In this article, we'll see some of the popular Matrix coding problems for coding interviews. I actually learned a lot by solving Matrix related problems. I still remember the first Matrix based problem I solve was about how to multiply two matrices in Java and I learned a lot about multi-dimensional array and nested loop in Java by solving that problem. The second problem I solved was about transposing Matrix and that was also quite challenging for me at that time but helped me to further solidify my knowledge about loops and array in Java. 

Every since then I have always looked for interesting Matrix based coding problems and today, I am going to share a couple of gems with you. You can use them to test your programming and coding skills.

If you are preparing for coding interview or looking for coding problems just want to improve your programming skills then you have come to the right place. In the past, I have shared coding problems on array, linked list, binary tree, dynamic programming, system design, recursion, and data structures in general and in this article I will cover Matrices. 

And, if you want to prepare really well, I also suggest you to join dedicated coding interview courses like Master the Coding Interview: Big Tech (FAANG) Interviews by Andrei Negaoie and Yihua Zhang on Udemy. This is a great resource for anyone preparing for coding interviews on those big companies like Microsoft, Google, and Facebook, and you can buy this course for just $10 during Udemy sale, this is like getting it free, one of the reason I love Udemy so much. 




10 Matrix based Coding Problems and Exercises for Practices

Without wasting anymore of your time, here is a list of Matrix based coding problems for practice. You can use these Matrix coding questions to not only prepare for programming job interviews but also for improving your coding skills. I have included both beginner, medium and difficult level Matrix questions in this list. Let me know how many you an solve without getting any help on Internet. 

1. Vertical Flip Matrix Problem

This is one of the less challenging Matrix problem to start with. In this question you will be given an m x n 2D image matrix where each integer represents a pixel and you need to write a method flipVerticalAxis() to flip it in-place along its vertical axis.

Examples
[[1, 0],
[1, 0]]
->
[[0, 1],
[0, 1]]


2. Horizontal Flip Matrix Problem 

This problem is a variant of previous Matrix based coding problem. In this questions you will be given an m x n 2D image matrix where each integer represents a pixel and you need to write a method flipHorizontalAxis() to flip it in-place along its horizontal axis. If you can solve previous problem about vertical flip then this would be easier, both question follow same pattern. 

Examples
[[1, 0],
[0, 1]]
->
[[0, 1],
[1, 0]]



3. Transpose a Given Matrix [Solution]

This is one of the easiest Matrix based problem you can get in any Coding interview. In this case, you will be given a square 2D image matrix where each integer represents a pixel. Write a method transposeMatrix() to transform the matrix into its transpose - in-place. 

One thing which is worth remembering that the transpose of a matrix is a matrix which is formed by turning all the rows of the source matrix into columns and vice-versa. In case you don't remember this concept, you should ask about it form Interviewer, there is nothing wrong on that. 

Examples
[[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 0, 1, 2],
[3, 4, 5, 0]]
->
[[1, 5, 9, 3],
[2, 6, 0, 4],
[3, 7, 1, 5],
[4, 8, 2, 0]]




4. Rotate a given Matrix in Java

In this Matrix based coding questions you will be given an image represented by an NxN matrix, where each pixel in the image is 4 bytes, and you need to write a method rotate to rotate the image by 90 degrees in place.

Examples
[[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 0, 1, 2],
[3, 4, 5, 6]]
->
[[3, 9, 5, 1],
[4, 0, 6, 2],
[5, 1, 7, 3],
[6, 2, 8, 4]]



5. Boggle Search Problem

This is again a popular Matrix related coding problem and often asked during coding interviews. You’re given a 2D Boggle Board which contains an m x n matrix of chars - char[][] board, and a String - word. Write a method - boggleSearch that searches the Boggle Board for the presence of the input word. 

Words on the board can be constructed with sequentially adjacent letters, where adjacent letters are horizontal or vertical neighbors (not diagonal). Also, each letter on the Boggle Board must be used only once.

Examples

Input Board :
[
[A, O, L],
[D, E, L],
[G, H, I]
]
Word: "HELLO"
Output: true

Top 10 Matrix Coding Problems for Programing interviews



6. Zero Matrix Problem

Write a method zeroMatrix with an algorithm such that if an element in a MxN matrix is 0, its entire row and column are set to 0. Matrix should be changed in place.

Examples
[[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 0, 1, 2],
[3, 4, 5, 0]]
->
[[1, 0, 3, 0],
[5, 0, 7, 0],
[0, 0, 0, 0],
[0, 0, 0, 0]]


7. Count Paths in Given Matrix

You’re given a game board that has m x n squares on it, represented by an m x n array. Write a method countPaths that takes in m and n and returns the number of possible paths from the top left corner to the bottom right corner. Only down and right directions of movement are permitted.

Examples
countPaths(m = 2, n = 2) => 2

as on the following 2x2 Board, the two paths are A->C->D and A->B->D

A B
C D




8. Matrix Max Sum Path Problem

Given an m x n matrix filled with non-negative integers, find the maximum sum along a path from the top-left of the grid to the bottom-right. Return this maximum sum. The direction of movement is limited to right and down.

Examples
[[1, 2, 3],
[4, 5, 6], -> 29 (1 + 4 + 7 + 8 + 9)
[7, 8, 9]]


9. Largest Square Matrix Problem

Given a two dimensional matrix made up of 0's and 1's, write a method largestSquare to find the largest square containing all 1’s and return its area.

The area is simply the sum of all integers enclosed in the square.

Example
Input Matrix :

1101 xxxx 11xx
1101 => 11xx or 11xx
1111 11xx xxxx

Output : 4


10.  Spiral Matrix Problem

In this problem, you are given an m x n matrix and you need to return all elements of the matrix in spiral order. This is a medium level Matrix problem.

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]] 

Output: [1,2,3,6,9,8,7,4,5]

Here is an example of Spiral order in Matrix:
 
Top 10 Matrix Problems for Programing interviews



That's all about common matrix based coding problems for programmers and developers. You can use these questions to not just prepare for coding interviews but also to improve your programming and coding skills. If you are a beginner then Matrix based problems are good to improve your knowledge about loops and multi-dimensional array. They are really good to improve your coding sense as well. 

Other Useful Resources for Coding Interviews:
Thanks for reading this article so far. If you like these Matrix coding and Programming problems and interview questions then please share them with your friends and colleagues. If you have any questions or feedback then please drop a note.

P. S. - If you need more practice, including dozens more coding problems and solutions for each pattern, check out Grokking Coding Interview Patterns  on Educative. It's an excellent, text-based interactive course to build your Dynamic programming skills. Educative as a platform is also a great resource for coding interviews and you can get all of their course access by just $14.9 per month. I highly recommend this to anyone preparing for programming job interviews. 

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