最长回文子串

class Solution:
    def longestPalindrome(self, s: str) -> str:
        if s == None:
            return None
        length = len(s)
        if length <= 1:
            return s
        dp = [[0 for i in range(length)] for i in range(length)]
        ss = s[0]
        re = 1
        for i in range(0,length):
            for j in range(0,i+1):
                if i-j<=1:
                    if s[j]==s[i]:
                        dp[j][i]=1
                        if re < i-j+1:
                            ss = s[j:i+1]
                            re = i-j+1
                else:
                    if s[j]==s[i] and dp[j+1][i-1]:
                        dp[j][i]=1
                        if re < i-j+1:
                            ss = s[j:i+1]
                            re = i-j+1
        return ss