Find Longest Palindromic Substring

Given a string, find the longest palindromic substring. The input is a string consisting of uppercase letters, and the output is the longest palindromic substring. If there are multiple substrings of the same length, return the lexicographically smallest one. Input: String: A string consisting of uppercase letters Output: The longest palindromic substring. Sample Input: YABCBAZ Sample Output: ABCBA Example Input YABCBAZ