leetcode-137-只出现一次的数字②

题目描述：

方法一：数学

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        return (sum(set(nums))*3 - sum(nums))//2

方法二：

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        hash_table = {}
        for i in nums:
            if i not in hash_table:
                hash_table[i] = 1
            else:
                hash_table[i] +=1
        for i,v in hash_table.items():
            if v == 1:
                return i
        

方法三：位运算*

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        res = 0
        for i in range(32):
            cnt = 0
            bit = 1 << i
            for num in nums:
                if num & bit != 0:
                    cnt += 1
            if cnt % 3 != 0:
                res |= bit

        return res - 2**32 if res > 2**31-1 else res