Being interviewee: Matrix spiral copy

April 26, 2018

Introduction

It is my most favorite algorithm called Matrix spiral copy. I told the peer that I like to write a solution to automate direction change but use extra array to store visited information.

I was nervous since last time I wrote the solution was months ago. I am kind of memorizing the solution. What I did is to use wisdom learned through 300 mock interview experience, I tell the peer that I like to visit first row 5 elements 1, 2, 3, 4, 5. How will I do it?

Mock interview

I had discussion with the peer and the peer questioned me the problem I had to visit first element 1 twice. I missed the reset of row and column variables from line 44 and line 45, and also line 42 to set visited array on nextRow, nextCol true.

How come I discussed the solution with the peer with 2 most important task missing? I told the peer that my code will pass all test cases.

I failed last test case and then I noticed that I need to add line 44 and line 45. I need to reset row and col variables. And then I still failed another test case, I noticed that I need to set visited array; and then I still failed the test case, I noticed that I need to set [nextRow, nextCol] instead of [row, col].

What a mock interview!

I need to structure my mock interview better, guard with the whiteboard testing.

Here is my C# code.

Good thing is that I spent less than 20 minutes to write the code with the analysis. The bugs is very easy to fix and code is minimum. It took me less than five minutes to fix all the bugs.

The challenge part is to write correct code and do whiteboard testing by myself.

Being interviewer: Leetcode 54: Spiral matrix

March 24, 2018

Introduction

I had over 14 interview experience to ask interviewee to work on Leetcode 54: Spiral matrix. I like to go over each practice and then review the code. I am too busy to try new algorithms, it is still very good to review my past experience and figure out something to learn.

Code review

Here is my Feb. 6 interview code I reviewed. The interviewee is an excellent programmer and also pass one of top 4 companies onsite interview. He showed me how good he is a developer. He was very busy writing code, run test, and keep good presentation. Overall the presentation is so nice.

I never experience such great presentation. I like to learn from his presentation skills. Overall the presentation of his code is perfect. I asked him to test one row and one column, and then he added a few lines of code before the while loop from line 48 to 52.

HackerRank NCR code sprint: Spiral Message

March 23, 2018

Introduction

I spent more than 30 minutes to review my own question on stackexchange.com, and I like to post an answer after 15 months. Here is the question I asked in January 2017.

Code review

I wrote the code again, here is the C# code. I will write an answer in short future.

1. change variable names to make it more meaningful.
 startX is changed to rowStart,
 startY is changed to colStart,
 endX   is changed to rowEnd,
 endY   is changed to colEnd.

2. Remove code related to check base cases like one node, one row, one column, set direction variables.

3. Add base case one node and one column break statement just after the first leftmost column.
4. Add base case one row break statement just after the second top row.

My last 3 month practice

The matrix spiral print is my most favorite algorithm, first in January I was interviewed and got low performance 1/ 4 problem solving rating. After that I tried to practice more than 10 ideas using Leetcode 54 discussion panel, I asked the question on stackexchange.com, and then i used the algorithm to interview more than 13 interviewees on mock interview platform. If I do learn something, I must learn from this algorithm.

Here are the links to look up. I still get organized by adding/ modifying the labels on blog platform.

Search by keyword: spiral matrix print, here is the link. Search using keyword: Leetcode 54: Spiral matrix, link is here.

Spiral matrix print algorithm

March 8, 2018

Introduction

It is my most favorite algorithm in 2018. I learn so many lessons from the algorithm, and I can count so many hours I have spent on the algorithm. My most favorite experience is documented here, one thing I did not tell that the first mock interviewer on anonymous platform in 2018 failed me on this algorithm and marked me as poor as the technical skills. I did not write fully functioning code following the idea using direction array and automate the direction change.

One more interview

I chose the algorithm to interview the peer on March 7, 2018 10:00 PM. The peer spent near 45 minutes to complete the algorithm.

Here is C# code I reviewed.

Even though I have worked on the algorithm so many hours, I did not perform very well as an interviewer yesterday. I preferred one while loop instead of three loops with while/for/while, I confused and made wrong judgement to say that for loop will have dead loop, actually the for loop direction array only has size of 4.

Actionable Items

I like to replay the mock interview I did on January 23, 2018 to work on spiral matrix print algorithm. After 30 days of mock interview, I like to compare myself with all my 13 interviewee to work on the same algorithm.

What should I work on to perform better through mock interview?

Leetcode 54: Spiral matrix

Feb. 12, 2018

Plan to study Leetcode 54: Spiral matrix python solution. Here is the link.

Leetcode 54: Spiral Matrix

Feb. 12, 2018

Introduction

It is 12:00 PM mock interview. I had to work on Spiral matrix algorithm. So I spent around 20 minutes to write a solution based on my last practice.

I learned the lesson on interviewing.io on January 23 2018. The lesson is that I have to take mock interview seriously. Follow the interviewer. If I am asked to find the alternative solution, then I have to fully use those 40 minutes to come out the solution. Work on it, and define the requirement, and address the issues, come out the design, techniques to solve the problems first.

Code review

Here is my C# code.

Leetcode 54: Spiral matrix - based on directions

Feb. 9, 2018

Introduction

I keep learning the spiral matrix algorithm through Leetcode discussion, and then I came cross the discussion of Leetcode solution based on the directions.

Here is the discussion note I like to share. I read carefully about the analysis and then write down and make some modification, so I can follow closely the thinking process of the author.

Analysis based on directions

I like to go over the discussion here as well. Here is the C# code. Also I like to add this solution to the question I asked on stackexchange.com so that I can get some feedback later on.

Leetcode 54: Spiral Matrix

Feb. 8, 2018

Introduction

One more solution is here based on using two variable to switch direction clockwise. I read the idea from Leetcode 54 discussion, and then tried to write one using C# language.

One of ideas is to go over as many Leetcode discussion as possible, and then I will vote the idea I like and actively share my feedback. Because I like to use this algorithm for my mock interview as an interviewer, I like to do as many research as possible.

Please write at least ten ideas using C#. Be well prepared and learn one algorithm very well. I can  use the algorithm to tell who is real talented programmer in the world.

Using two direction variables

Here is C# code.

Leetcode 54: Spiral matrix - think recursively

Feb. 8, 2018

Introduction

It is my idea how to train myself in the practice. It is to involve the discussion of Leetcode. Every algorithm I should give out 10 votes for 10 different ideas, and also write C# code based on the idea.

I did some research based on my practice of Leetcode 301 recently, and I found out that my practice has some issue. And I wrote a blog how to learn a hard level algorithm on Leetcode.com.

Recursive solution

I studied the solution today, and I like to write a C# code based on the idea. It is the great warm up of recursive function, and also nice to review base cases in the design of recursive function. Here is the C# code.

spiral matrix algorithm

Feb. 7, 2018

It is my decision to start to work on Leetcode 54 Spiral matrix again. I like to go over all possible solutions through Leetcode discussions, and then vote some of them, and write C# version.

One thing I can do is to work on the same algorithm but use various approach. And I need to understand what the difference is, what makes impact on time consumption on writing, easy to avoid edge cases handling, and what is the production code.

After I practiced over 10 times on the algorithm starting from March 2017 to January 2018, I found the weakness of my practice.

Leetcode 54: Spiral matrix

Feb. 2 2018

Introduction

It is the second time I asked leetcode 54: spiral matrix in the anonymous interview as an interviewer. I met a peer who was willing to be interviewed, and followed my hint and try to write a solution using one loop instead of four for loops for each direction.

Code review

Here is Java code to review. The peer spent around 20 minutes to work on coding, I was able to see how good the peer can write readable code.

I told the peer that he did better than me. When I was asked this algorithm and the idea to write one loop only, it took me more time to come out the ideas, and I needed two hints, one for direction array, one for visited array.

Compared to the peer, I knew that I have to learn to stay open and think hard in the mock interview.

Leetcode 54: Spiral matrix

January 30, 2018

Introduction

It is such a big surprise that my first mock interview as an interviewer turns out big success. The peer solved all four algorithms I asked in first 40 minutes. He is the best peer I have met in my peers, over 150 peers.

Best thing is that I got education how to design a perfect system design: Instagram scalability in 30 minutes. I could not believe that I may interview a top company senior developer in the future.

Code review

Here is the C# code I like to learn. The idea is very clear and it is the production ready code.

Leetcode 54: Spiral Matrix

January 28, 2018

Introduction

It is the algorithm I have to write on my 10:00 PM mock interview. I worked on the spiral matrix, and it took me in total 20 minutes to explain the solution and write the solution to pass all test cases.

Code review

The peer told me that I can save the space using four pointers instead of visited array. I explained to him that the mock interview is for me to write the correct solution using minimum time.

Here is the C# code I wrote.

Leetcode 54: Spiral Matrix

January 24, 2018

Introduction

It is the algorithm I have practiced over 6 times in mock interview and also interviewed other 6 peers from March 2017 to January 2018. On January 23, 3018 10:00 pm, I had my first mock interview on interviewing.io platform, and then I had to work on the algorithm. What I learned through the mock interview is to get rid of those 4 for loop inside a while loop. In other words, the interviewer told me that in order to perform a 20 minutes coding interview, I have to write only one loop, and adjust the direction accordingly.

Mock interview 

Here are the hints the interviewer gave to me:

1. After 20 minutes, he wrote down four directions in two one dimension array.
dr = new int[]{0, 1, 0, -1}
dc = new int[]{1, 0, -1, 0}

2. After another 10 minutes, he gave second hint to use visited array [][] to mark the visited node.
3. After that, he gave me another hint, how to write a checking for direction change.

if(row < 0 || row >= rows || col < 0 || col >= cols || visited[row][col] == 1)
{
      // change direction
}

Here is the transcript I copied from video clip at 50:00 of 51:00.

After the mock interview 

This the first time I used interviewing.io to practice mock interview. It turns out that the interviewer is much strong in technical skills and also interview skills. I have played the interview recording twice, and here are something I have to work on.

1. Do not interrupt the interviewer.
2. Say something like "Let me think about two minutes.". Actually calm down and think carefully in two minutes.
3. Stay calm and speak slowly.
4. Take hint and write down the hint quickly, and really think about hint. Follow the hint.
5. It is too slow to take the hint, and work out a solution based on the hint.
6. I should work on the example and write fully functional C# code in mock interview.
7. Check the time I spend and act fast.

Define the keywords for the problem, write down in mock interview. So it is very easy to discuss with the peer for various ideas. Seek and you will find, brainstorm the ideas related to time complexity, space complexity.

Direction - There are four direction. Change direction if need, in the order of clockwise, starting from top left corner (0,0).
Range - Stay inside the array
Visit - visit each element in the array only once. Do not visit more than once.
Order - follow the order of clockwise, start from (0,0).

Ideas can be searched through those keywords. Let us go over one keyword together.

For example, direction can be managed smartly using array for four direction, or manually specified. Every edge in one direction can be specified by start point and end point two variables, but it also can be implicitly specified using visited array. Just relax and talk to the peer in mock interview, and go over each possible options. Evaluate pros and cons, have a conversation.

Follow up 

Here is the C# code I wrote after the mock interview. I really like the idea and the solution is so easy to write, I write on purpose to add fourDirections in order to make the code more readable.

I am planning to post a code review on the stackexchange.com as well. The link is here.

Feeback is gold

It takes a village to raise a child. 

Interviewing.io is my new village. 

Leetcode 54: Spiral Matrix

Sept. 3, 2017

Introduction

It is the classical medium level algorithm to write spiral matrix in one dimensional array. I have practiced more than 3 times. The key idea is to go over the four iterations from top row, last column, last row and first column, edge cases are one row or one column. last row and first column is optional because they are handled only if there are more than one row or more than two columns.

Compared to the practice in March of 2016, I am so happy to see that I can write more readable code.

Algorithm practice 

C# code is here. 

Feedback 

Leetcode 54: Spiral Matrix

May 23, 2017

Introduction

It is a good habit to review the code and then run a few test case to verify the code. Once you finish the white board testing, you tell that you are ready and code is complete. As a software programmer, it is not too late to start to build a habit, examine the code with coding style, run white board testing, and think about possible bugs. Have a high standard, push yourself.

Code design talk

C# code submission is here. Leetcode discussion link is here. Plan to ask a question on codereview.stackexchange.com.

Build a ritual to do white board testing, and write down the test case next to each line of code.

Mock interview (4th practice): Matrix Spiral Print

March 16, 2016

Problem statement

Given a 2D array (matrix) named M, print all items of M in a spiral order, clockwise.

For example:

M  =  1    2    3    4   5
         6    7    8    9  10
       11  12  13  14  15
       16  17  18  19  20

The clockwise spiral print is:  1 2 3 4 5 10 15 20 19 18 17 16 11 6 7 8 9 14 13 12

Mock interview practice

I had a mock interview recently and my algorithm is Spiral matrix. The code I wrote in mock interview using C# is here.

Let us take a look at evaluation from the mock interviewer, as a matter of fact the peer gave me the honest feedback, I have to figure out how to work on the improvement:

Code review

After the mock interview, I thought about more how to improve the solution. I do not have good idea to solve the problem. Based on my mathematics background and I tried to define how many variables in the problem.

Let us look at one variable, let us call it layer, using variable name row, value is from 0 to (N+1) / 2, where N is how many columns in the matrix. So, the spiral matrix output is to follow the order of clockwise starting from (0,0). 

N - how many columns

M - how many rows

Four corners are left-top, right-top, bottom-right, bottom-left, abbreviation using four variables: LT, RT, BR, BL

   LT     coordinates:   ( row,             row )    

   RT    coordinates:   ( row,             N - 1 - row)  

   BR    coordinates:   ( M - 1 - row,  N -1 - row)

   BL     coordinates:   ( M - 1 - row,  row)

private static void leftToRight(Coordinate[] A)

TopToDown, RightToLeft, and DownToUp 

  

           row

            0     1       2

            -------------->

           1    2     3   4   5

            6    7     8   9  10

           11  12  13  14  15

           16  17  18  19  20

The above case, LT = (0, 0), RT = (0, 4), BR = (3, 4), BL = (3, 0)

C# practice code based on the above idea is here.  

Continue to improve the idea. 

LT, BR those two pointers should be checked on the conditions, M – 1 - row >= row, N – 1 - row >= row; in other words, left top pointer is above the bottom right pointer, therefore it should be row <= Math.Min((M - 1)/ 2, (N - 1)/ 2).

It is true that the idea I came out just after the mock interview was not so good. But the hard work spirit is good, and also it is very good to write down the idea and review later on.

Issues found in mock interview

I like to write down a few issues in my practice. 

1. Jagged array initialization

Julia spent more than 5 minutes to look up the internet. 

2. Try another idea. Use one variable instead of two variables. 

Assuming that N rows and M columns, how many layers of spiral? Use the variable i to denote the number. 

It should be from 0 to (N+1)/2, but it is hard to figure out correct answer first time, it should be 0 to Math.Min((M-1)/2, (N-1)/2). 

Julia spent more than 20 minutes to debug in the practice. 

Spiral matrix practice in 2015

I practice once in 2015 on Leetcode 54 Spiral matrix algorithm. I found the blog and it is very interesting to read the code I wrote back in 2015 June. Here is the blog about the practice. 

Follow up 

May 23, 2017

It is such great experience to compare the current practice to the one in 2016. Julia learned that she made such great improvement on coding readability, and she starts to know how important it is to document and track the progress. 

After considering a few of options, it is much easy to write a while loop to track the total visited nodes in the original array. 

Instead of using i, j, use variable row and col because it is more meaningful. 

Do not run the code and depend on the compiler; 

Test the code by yourself. 

C# practice is here. 

In order to make sure that the code is working, Julia used the same idea to write a solution for Leetcode 54. The solution failed to pass one row test case [1, 2, 3]. 

The C# code is fixed to add one row and one column checking. The code is here. 

Leetcode 54: Spiral Matrix | Code review | 2015 - 2023 | Long journey

June 11, 2015

Problem Statement

Recursive solution

The first study code is here. The second is here.

Iterative solution

The first study code is here. The second one is here. 

C# code to pass Leetcode online judge:

Julia's C# code practice in 2015

Follow up 

May 23, 2017

C# practice passes all test case. The code is here. Put one row and one column checking inside the for loop statement. Code is here.

Feb. 13, 2018

It is the early day I started to write coding blog to document my code practice. At that time, I was so shy and also afraid to write down my thinking process. I do not know myself even it is just three years ago.

I just wrote down two sentences, even the sentence is not complete. I chose to study two solutions and then I practiced one of them.

One thing I can tell now is that I did not pay attention to myself, how I think as a programmer, most likely I thought that coding is more important and also challenge.

The blogs I chose in 2015

Those algorithm blogs are well-written. I am surprised that I did not practice one by one in 2015.

The recursive solution is saved in the gist. Here is the link.
The solution based on directions is saved in the gist. Here is the link.
The solution based on directions and range is here.

Follow up 

July 4, 2023

I am so lucky to have chance to prepare for another phone screen from Meta in short future. I have chance to review my work history on this algorithm.