Longest Self-Sufficient Proper Substring

amazon In natural language processing preprocessing, you need to handle strings to find the length of the longest self-sufficient proper substring. The substring cannot be the whole string itself. Characters in the substring cannot appear outside the substring. Given a string fullString consisting of lowercase letters with length n, return the length of the longest substring meeting the above conditions. If no such substring exists, return 0. Input: fullString = "abcabcbb" Output: 0 Constraints: 1 <= fullString.length <= 10^5 Example Input abcabcbb