R7-多维dp篇
思路：
两种情况，中心1个数，中心2个数
大体：中间点扩展原则
class Solution:def expand(self,s,left,right):while left>=0 and right<len(s) and s[left]==s[right]:left-=1right+=1#不满足就回退return left+1,right-1def longestPalindrome(self, s: str) -> str:start,end=0,0for i in range(len(s)):start1,end1=self.expand(s,i,i)start2,end2=self.expand(s,i,i+1)if end1-start1>end-start:start,end=start1,end1if end2-start2>end-start:start,end=start2,end2return s[start:end+1]
