Colouring a Sequence of Boxes
One of my friends was applying to Google for a job and got this problem in one of the final written rounds. This was a fun little problem that helped me procrastinate...
Problems
Some Problems in Linear Algebra
I came across a couple of fun problems when I was going through "Linear Algebra Done Right" by Axler. I really enjoyed this book and it was great for a "second reading" of the basics of Linear Algebra.