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: ccnnmncccmcn
Example Output: nmn

Example Input: grqqqrqrqgggrg
Example Output: rqqqr

Example Input: zzzkzznknnzz
Example Output:
zzkzz