最长回文子串

class :
    def longestPalindrome(self, s: str) -> str:
        def isplain(newlist=[]):
            flag=False
            length=len(newlist)
            mid=length//2
            for i in range(mid):
                if newlist[i]==newlist[length-i-1]:
                    flag=True
                else:
                    flag=False
                    break
            return flag
        length=len(s)
        if length<2:
            return s
        allStrings=[]
        count=0
        result=''
        flag=False
        for idx,i in enumerate(s):
            for j in range(length-1,idx,-1):
                if j-idx<=count-1:
                    break
                if i==s[j]:
                    newlist=s[idx:j+1]
                    if isplain(newlist):
                        strlen=len(newlist)
                        if strlen>count:
                            count=strlen
                            result=newlist
                            flag=True
        if flag is False:
            result=s[0]
        return result