Citadel Software Engineer Interview Experience — Two First Rounds and Progressive Matrix Questions

I asked a lot of people, and they all said they had two first-round interviews. I received both as well, but I did not make it to the second round.

The first one:

I was asked only one question: share some especially difficult problems you worked on recently and explain them.

For the coding portion, a prefix tree or trie was enough to solve it. The question was very similar to a search suggestion system.

The second one:

The interviewer asked very detailed questions about my resume and projects, so make sure to prepare those carefully.

He said I should have three coding questions, but I only reached the second one. I did not have time to finish writing the second because I spent too much time on the earlier part. The three coding questions built on one another.

The first was a simple 2D matrix traversal, except that the boundary rows and columns could not be changed.

The second required calculating, for every cell in a 2D matrix, the sum of the corresponding k-by-k submatrix. It needed a 2D prefix sum.