Given a string s, return all the palindromic permutations (without duplicates) of it. Return an empty list if no palindromic permutation could be form.
Hint 1
If a palindromic permutation exists, we just need to generate the first half of the string.
Hint 2
To generate all distinct permutations of a (half of) string, use a similar approach from: Permutations II or Next Permutation.