code javascript: Given a string, return the longest substring in which every letter occurs an even number of times.

Task description

Write a function:

function solution$($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 $0.$

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 $(\text{'}$a$\text{'}-\text{'}$z$\text{'}).$