The first solution I came up with and saw that it has not been posted here yet. It was accepted but is not a O(log n) solution. Perhaps something needs to be done so that such a solution will not be accepted? More generally, is this an efficient approach if the O(log n) constraint is not specified?
class Solution(object): def singleNonDuplicate(self, nums): """ :type nums: List[int] :rtype: int """ for n in set(nums): if (nums.count(n)!=2): return n