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class Solution:
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    def singleNonDuplicate(self, nums: List[int]) -> int:
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        return self.b_search(nums)[0]
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    def b_search(self, nums):
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        if len(nums) == 1:
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            return nums
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        mid = len(nums) // 2
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        a = nums[:mid]
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        b = nums[mid:]
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        # check if last & first element of the two sub lists are same
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        if a[-1] == b[0]:
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            a = a[:-1]
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            b = b[1:]
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        # ignore the sub list with even number of elements
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        if len(a) % 2:
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            return self.b_search(a)
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        else:
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            return self.b_search(b)