用更加好懂的状态机做

class Solution(object):
    def findNumberAppearingOnce(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        ones=0
        twos=0
        for n in nums:
            one=ones
            ones=(ones&(~twos)&(~n))|((~ones)&(twos)&(n))
            twos=((~one)&(~twos)&(n))|((~one)&(twos)&(~n))
        return twos