text: class Solution:
      def longestNiceSubstring(self, s: str) -> str:
      	"""Divide and conquer.
      	"""
          if not s:
              return ""
          sset = set(s)
          for i in range(len(s)):
              if s[i].upper() in sset and s[i].lower() in sset:
                  continue
              lsub = self.longestNiceSubstring(s[:i])
              rsub = self.longestNiceSubstring(s[i+1:])
              return lsub if len(lsub) >= len(rsub) else rsub
          return s
  