Given a string S, find the length of the longest substring without repeating characters.
Example 1:
Input:
S = "geeksforgeeks"
Output:
7
Explanation:
Longest substring is
"eksforg".
Example 2:
Input:
S = "abdefgabef"
Output:
6
Explanation:
Longest substring are
"abdefg" , "bdefga" and "defgab".

Your Task:
You don't need to take input or print anything. Your task is to complete the function longestUniqueSubsttr() which takes a string S as and returns the length of the longest substring.

Expected Time Complexity: O(|S|).
Expected Auxiliary Space: O(K) where K is constant.
Constraints:
1 ≤ |S| ≤ 10^{5}
It is guaranteed that all characters of the String S will be lowercase letters from 'a' to 'z'

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