I'd had a "Computer science for electrical engineers" course in school, which discussed data structures and algorithms from a high level (the usual sorting algorithm discussion, implementing a linked list, that kind of thing), but nothing too in-depth. I've had various experience in programming before PE.

In solving PE problems I've mostly used Wikipedia and Mathworld for research, and sometimes I'll Google for lecture notes on a relevant topic.

I've used the Python skills I've picked up from PE in my job already. I think I could function in a more programming-oriented job now, though solving math problems doesn't give you much help in hooking into existing APIs or writing web services, which are probably pretty important.