leetcode 只出现一次的数字

class Solution:
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        
        stack = []
        for i in nums:
            if i not in stack:
                stack.append(i)
            else:
                stack.pop(stack.index(i))
        print(stack[0])