def longest_SubPal(self,s):
    """
    : type s: str
    :rtype:str
    """
    if not s: return s
    res = ""
    for i in range(len(s)):
       j=j+1
          while j<=len(s) and len(res)<=len(s[i:]):
             if s[i:j] == s[i:j][::-1] and len(s[i:j]) > len(res):
                res=s[i:j]
                j=j+1
                return res