Write a function to find the longest palindromic substring in a given string.

**Expand Around Center:** For each center (char or between chars), expand while palindromic. O(n²) time, O(1) space. Manacher's = O(n) but complex. def longest_palindrome(s): def expand(l, r): while l >= 0 and r < len(s) and s[l] == s[r]: l -= 1; r += 1 return s[l+1:r] return max((expand(i,i) or expand(i,i+1) for i in range(len(s))), key=len, default='')