a function class Solution $($public int solution$($String S$)$; $\}$ that, given a string S consisting of N lowercase English letters, returns the length of the longest substring in which every letter occurs an even number of times. A substring is defined as a contiguous segment of a string. If no such substring exists, return $0.$ A Examples: $1.$ Given S "bdaaadadb", the function should return $6.$ Substrings in which every letter occurs an even number of times are $"$aa$",$ "adad", "daaada" and "aaadad". The length of the longest of them is $6.2.$ Given S $=$ "abacb", the function should return $0.$ There is no non$-$empty substring in which every letter occurs an even number of times. $3.$ Given S $"$zthtzh$",$ the function should return $6.$ Every letter in the whole string occurs an even number of times. Write an efficient algorithm for the following assumptions: $$ N is an integer within the range $[1.\mathrm{.}100,000),$ string S consists only of lowercase letters $($az$).$