Digit factorial chains
This problem is interesting. It is not difficult to brute force. But finding a very efficient algorithm is not that easy. My code runs 0.7second without too much memory cost. The key is to avoid duplicate computation.
PE problem makers like factorial quite a bit. There are quite a few more difficult problems that are related with factorials.
update 09/25/2017:
the old code is still hard to read. I rewrote the recursive function to find the different numbers in the list. The logic is easier to understand now.
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