In this Kata, you will be given a string and your task is to return the most valuable character. The value of a character is the difference between the index of its last occurrence and the index of its first occurrence. Return the character that has the highest value. If there is a tie, return the lexicographically lowest character.
All inputs will be lower case.
solve('a') = 'a'
solve('ab') = 'a'. Last occurrence is equal to first occurrence of each character. Return lexicographically lowest.
solve("axyzxyz") = 'x'