longest palindromic substring.py

class Solution:
    def longestPalindrome(self, s: str) -> str:
        res = ""
        for i in range(len(s)):
            res = max(self.helper(s, i, i), self.helper(s, i, i+1), res, key=len)
        return res
    
    def helper(self, s, l, r):
        while 0 <= l and r < len(s) and s[l] == s [r]:
            l -= 1; r += 1
        return s[l+1:r]