text:  class Solution:
      def XXX(self, s):
          """
          :type s: str
          :rtype: str
          """
          str = ""
          for i in range(2*len(s)-1):
              if i%2 == 0:
                  start = end = i//2
                  while start>=0 and end<len(s) and s[start]==s[end]:
                      start-=1
                      end+=1
              else:
                  start = (i-1) // 2
                  end = (i+1) //2
                  while start>=0 and end<len(s) and s[start]==s[end]:
                      start-=1
                      end+=1
              if len(str)<=(end-start-1):
                  str=s[start+1:end]
          return str
  
  