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.