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: yayhaayyyyh
Example Output: yyyy

Example Input: gggggxffggfxff
Example Output: ggggg

Example Input: tvvxvtvtxxxvvvt
Example Output:
vxv