project euler problem 095

Amicable chains

I checked my old code. It was cumbersome. I used set and my algorithm to find all divisors was too slow. It takes 6.5 second to find the answer. 

This time, I changed the divisor sum algorithm by using some sieve method. But it is still not very fast since I used hashset to check if the number appeared in the sequence. It is kind of overkill. But this reduced the run time to 0.764 second. 

I read the discussion thread and find a better algorithm to stop the sequence. I reimplemented the code, it reduced to 0.196 second with my debug version. Sweet!