120-136. 只出现一次的数字

问题描述:
      给定一个非空整数数组，除了某个元素只出现一次以外，其余每个元素均出现两次。找出那个只出现了一次的元素。

解释:其中前三种是我写的,后面两种是抄的
class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        set_nums = set(nums)
        for item in set_nums:
            if nums.count(item) == 1:
                return item
        return -1

    def singleNumber1(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        ret_dict = {}
        for item in nums:
            count = ret_dict.get(item, 0)
            if count == 1:
                del ret_dict[item]
                continue
            ret_dict[item] = count + 1
        return list(ret_dict.keys())[0]

    def singleNumber2(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        for i in nums:
            if nums.count(i) == 1:
                return i

    def singleNumber3(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        ret = 0
        for i in nums:
            ret = ret ^ i
        return ret

    def singleNumber4(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        return sum(set(nums))*2 - sum(nums)


if __name__ == '__main__':
    s1 = Solution()
    nums = [4, 1, 2, 1, 2]
    print(s1.singleNumber4(nums))