Problem 210 is not hard, it is easy to figure out what to compute and how to compute it. However, since the answer is more 18 digits long, and many computations require precision in both end (sqrt need to know digits after decimal and square numbers need to know the 18th digits). Ordinary computation will not work. This seems to be the trend of Project of Euler now. You get the idea quickly but spend a lot of time to deal with the small nuance. Not worth it. I used to tell myself once I solved all problems before 200, I will quit. Now, I have solved more than that, finished all problems except the last two. It's time to quit.

I have learnt a lot in PE, a really interesting project.

Other people are still encouraged to discuss PE problem in this thread.