回文是什么，回文是正着读和反着读都是一样的字符叫着回文。
 如 ‘aba’，‘aa’，‘b’，这些都是回文

class Solution:def longestPalindrome(self,s: str) -> str:n = len(s)dp = [[False] * n for _ in range(n)]ans = ""for l in range(n):#l代表1个字符串，2个字符串，3个字符串  for i in range(n):j = i+lif j >= len(s):breakif l == 0:dp[i][j] = Trueelif l ==1:dp[i][j] = (s[i]==s[j])else:dp[i][j] = (dp[i+1][j-1] and s[i]==s[j])if dp[i][j] and l+1>len(ans)ans = s[i:j+1]return ans