It was a 10:00 AM mock interview. I met a peer who is super talented programmer and also very good mentor to write clean code and also readable code. He helped me to think about design along the 30 minutes mock interview.
Here is transcript for C# code and with analysis. I did spend first 10 minutes to write nicely an analysis for the algorithm.
The code I wrote cannot pass the test case to find the interval, and then I could not find the logic problem. The peer told me on line 7 and line 8, I should check GetLength(0) < 1 instead. He said that he did not know C# very well, but he thinks that the checking is not necessary.