Fenwick Trees (which, despite the name, are implemented using an array) allow counting prefix sums AND updating prefix sums in O(log n) time. Very useful when n is in the order of millions. I have used them a few times in Project Euler problems.
Using about 10% of your mental capacity for your job is absolutely a good thing, if you're still doing what your company expects from you. This means that you still have 90% for yourself, and that's awesome.
If you don't feel challenged enough, seek intellectual activities (of ANY kind, as long as you enjoy it. Code, study, read, play an instrument, whatever) outside of your job. They will be much more satisfying, because you will have complete control over when and how do you engage in them. I've been doing this for many years and it's one the most satisfying aspects of my life, if not the most.
Also, being kind of a veteran (35yo), let me tell you this: be careful what you wish for. You seem to be enjoying a job that doesn't stress you or burn you out. If you start working at a company where you don't have that any more, there is a very good chance that you will miss the sustainability (in terms of mental health) of your current one.
TLDR: if your job pays the bills and doesn't offer challenges, great. Look for challenges in other areas of your life to maximise happiness.
https://en.wikipedia.org/wiki/Fenwick_tree