In this task you will be given a string and you should find the longest substring that is a palindrome. A palindrome is a string that is the same backwards as it is forwards. If the shortest possible palindrome is length 1 you should return the first character.

Example Input: avvvvaaaaavvzzv
Example Output: vvaaaaavv

Example Input: sssrrkkrrrssrk
Example Output: rrkkrr

Example Input: tttztzttzzzz
Example Output:
ttztztt