Longest Palindromic Substring

class Solution:
    def longestPalindrome(self, s: str) -> str:
        start, length = 0, 0

        def expand(left: int, right: int) -> None:
            nonlocal start, length
            while left >= 0 and right < len(s) and s[left] == s[right]:
                if right - left + 1 > length:
                    start = left
                    length = right - left + 1
                left -= 1
                right += 1

        for i in range(len(s)):
            expand(i, i)
            expand(i, i + 1)
        return s[start : start + length]