expand centers

class Solution:

    def expand_around_center(self, l, r, s):
        n = len(s)
        while l >= 0 and r < n and s[l] == s[r]:
            l -= 1
            r += 1

        return r - l - 1

    def longestPalindrome(self, s: str) -> str:

        n = len(s)
        if n < 1:
            return ''

        longest_palindrome = ''
        for i in range(n):
            len1 = self.expand_around_center(i, i, s)
            len2 = self.expand_around_center(i, i+1, s)
            max_len = max(len1, len2)
            if max_len > len(longest_palindrome):
                start = i - (max_len -1) // 2
                end = i + max_len // 2
                longest_palindrome = s[start:end+1]

        return longest_palindrome