leetcode 137. Single Number II

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题目链接：leetcode

思路：
使用xor异或的性质可以常数空间复杂度解决问题。

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class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        ones = twos = threes= 0
        for num in nums:
            ones = (ones ^ num) & ~twos 
            twos = (twos ^ num) & ~ones 
        return ones
print(Solution().singleNumber([1,1,1,2,2,2,3]))