The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.
Find the sum of the only eleven primes that are both truncatable from left to right and right to left.
NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
I believe that the way to approach this is to find a HashSet of all
right-truncatable prime numbers and then find which ones are
left-truncatable.
This depends on the idea that the number of right-truncatable primes is finite, whereas I'm not certain that the number of left-truncatable primes is.
...
Well, I needed help from the rust discord channel to tell me about why
HashSet iterators are iterators of &Item instead of being
iterators of just Item. Fortunately, they also told me about
.into_iter(), which produces an iterator of the underlying value
type (and destroys the collection, but that's fine where I use it)