def singleNumber(self, nums: List[int]) -> int:
    a = set()
    for i in nums:
        if (i in a):
            a.remove(i)
        else:
            a.add(i)
    for i in a:
        return i
def singleNumber(self, nums: List[int]) -> int:
    a = set()
    for i in nums:
        if (i in a):
            a.remove(i)
        else:
            a.add(i)
    for i in a:
        return i