Ask HN: Where to find programming challenges for sharpening algorithmic skills?

I've worked a bit with CodeChef and SPOJ. Both have problems that test problem solving abilities of varied nature.

However, I am unable to decide which one to follow through with. So I decided to ask the community.

What resources would you suggest for improving my thinking/algorithmic skills? Something, that'll have problems spanning all the fundamental concepts.

I am a student of Computer Science Engineering, and I'm not new to programming or algorithms; though I'm a far cry from being an expert.

I had tried my hand at Project Euler too, but you can get through those problems with sub-optimal solutions. For eg. the first problem's solution can be arrived by at by using an O(1) algorithm, but the system would accept a solution generated using an O(n) algorithm too. That wouldn't motivate deep thinking. (http://projecteuler.net/problem=1)

Will surely give it a whirl.

Each challenge takes the form of a word problem. The solution can be coded in a few different languages and uploaded to the website.

The site imposes time limits when running the solution. When I've found that my solution works but is too slow, it has encouraged me to look for different data structures or algorithms.

I can't recommend it enough.