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import math
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class Solution(object):
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    def singleNumber(self, nums):
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        """
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        :type nums: List[int]
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        :rtype: int
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        [0,-1,1,0]
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        [2,2,3,2]
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        [2,2,3,2]
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        [2,2,3,2]
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        10   10   11  10
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        """
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        total = sum(nums)
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        uniqueTotals = set()
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        while nums:
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            uniqueTotals.add(nums.pop())
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        return (sum(uniqueTotals) * 3 - total) / 2