Before I started Dev Bootcamp, one of the ways I prepared myself was by spending quite a lot of time solving various challenges, and it wasn't long before I came across Project Euler. I wasn't able to get too far with them, maybe only the first dozen or so, as the difficulty curve rose really quickly for me, and my 3 levels of Calculus in college weren't coming back to me at all easily.
One of the most memorable challenges I did for Project Euler was finding the sums of all the prime numbers below 2 million. This was a difficult problem for me because my brute-force solution—checking each consecutive number for primeness by taking its factors—was just incredibly slow, and therefore disqualified as a solution. (And by slow I mean it would have taken days.) I finally caved in and created a sieve of Eratosthenes as StackOverflow commentors suggested. To do that, I spent quite a lot of time staring at the lovely animation on the Wikipedia page for the sieve. I understood that the sieve didn't check for primeness; it instead assumed that multiples of given nums were already not going to be prime. Thanks to that, I finally came up with my solution for the prime problem: