you are given a string consisting of digits. Find the biggest two-digits value that is a consistent fragment of thr given string.

For example, two-digit consistent fragment of "50552" are["50","05","55","52"], representing the numbers [50,5,55,52]. the biggest value among them is 55.

Write a function :

class Solution { public int solution(String S); }
that, given a string S consisting of digits, returns the maximum two-digit value thst is a consistent fragment of S