Solid software engineer, but want to learn more algorithm-y type skillsets.

So recently I applied to a popular online freelancing platform and out of the three questions I was asked, I could only solve 1 and the other halfway there. I'm sure the solved one scored half or less of the full mark because of performance issues or whatnot. When I was in college I went to the ACM ICPC competition and these interview questions on Codility were right up that alley. Find smallest palindrome in large set, etc etc you get the picture. --- What book would you recommend for reading these questions, and dissecting solutions to really grok what o(N) really means? I've been programming for 7 years now and make a great living but when it came to these interview questions I was just flabbergasted, I've never had to do something like this. But hey it was fun, and I want to learn/practice more of these. It will definitely make me a better software engineer. :) Thanks for your suggestions guys/gals.