text:  class Solution:
      def XXX(self, prices: List[int]) -> int:
          if prices.__len__() == 0: return 0
          ret = 0
          pre = 1e4
  
          for i in range(0, len(prices)):
              if prices[i] < pre:
                  pre = prices[i]
              else:
                  ret = max(ret, prices[i] - pre)
  
          return ret
  
  
  
  
  