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class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        ls=[]
        x=0
        counter=defaultdict(int)
        for num in nums:
            counter[num] += 1
            if counter[num]>1:
                ls.append(num)      
        nls= list(set(nums)-set(ls))
        for i in nls:
            x=i
        return x

题解：异或运算 

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class Solution:
    def singleNumber(self, nums: List[int]) -> List[int]:
        x = 0
        for num in nums:  # 1. 遍历 nums 执行异或运算
            x ^= num      
        return x;         # 2. 返回出现一次的数字 x