new_contents: class Solution:
      # @param A, a list of integer
      # @return an integer
  
      def singleNumber(self, A):
          one = two = 0
          for a in A:
              two |= (one & a)
              one ^= a
              not_three = ~(one & two)
              one &= not_three
              two &= not_three
          return one
  
subject: add solution for Single Number II