A DNA lab flags "mirrored" stretches: substrings that read the same forwards and backwards. Given a string s, return the longest such substring. If several have the maximum length, return the one that starts first. For an empty string return "".
Examples
Input: s = "abacdc"
Output: "aba"
Explanation: "aba" and "cdc" both have length 3; "aba" starts first.
Input: s = "xyz"
Output: "x"
Constraints
0 <= len(s) <= 2000, lowercase letters only- Target: O(n^2) time, O(1) extra space.
Goals
- Enumerate both odd and even palindrome centres
- Expand outwards while the characters match
- Track the earliest longest result